Removed GrpcClientBuilder#MultiClientBuilder (apple#1809)

Motivation:

`GrpcClientBuilder#MultiClientBuilder` was previously deprecated and is no longer necessary.

Modifications:

- Removed `GrpcClientBuilder#MultiClientBuilder`,
- Removed `GrpcClientBuilder#buildMulti` method.

Result:

Unnecessary means to create multiple `GrpcClient`s has been removed and
there is one way to build `GrpcClient`s.

servicetalk-grpc-api/src/main/java/io/servicetalk/grpc/api/GrpcClientBuilder.java

@@ -386,38 +386,6 @@ public final GrpcClientBuilder<U, R> appendHttpClientFilter(Predicate<StreamingH
         return clientFactory.newBlockingClientForCallFactory(newGrpcClientCallFactory());
     }
 
-    /**
-     * Returns a {@link MultiClientBuilder} to be used to create multiple clients sharing the same underlying transport
-     * instance.
-     *
-     * @return A blocking <a href="https://www.grpc.io">gRPC</a> client.
-     * @deprecated Use {@link GrpcClientFactory#newClient(GrpcClientCallFactory)}
-     * or {@link GrpcClientFactory#newBlockingClient(GrpcClientCallFactory)}
-     * and provide a custom {@link GrpcClientCallFactory} instead.
-     */
-    @Deprecated
-    public final MultiClientBuilder buildMulti() {
-        GrpcClientCallFactory callFactory = newGrpcClientCallFactory();
-        return new MultiClientBuilder() {
-            @Override
-            public <Client extends GrpcClient<?>,
-                    Filter extends FilterableClient, FilterableClient extends FilterableGrpcClient,
-                    FilterFactory extends GrpcClientFilterFactory<Filter, FilterableClient>> Client
-            build(final GrpcClientFactory<Client, ?, Filter, FilterableClient, FilterFactory> clientFactory) {
-                return clientFactory.newClient(callFactory);
-            }
-
-            @Override
-            public <BlockingClient extends BlockingGrpcClient<?>,
-                    Filter extends FilterableClient, FilterableClient extends FilterableGrpcClient,
-                    FilterFactory extends GrpcClientFilterFactory<Filter, FilterableClient>> BlockingClient
-            buildBlocking(
-                    final GrpcClientFactory<?, BlockingClient, Filter, FilterableClient, FilterFactory> clientFactory) {
-                return clientFactory.newBlockingClient(callFactory);
-            }
-        };
-    }
-
     /**
      * Create a new {@link GrpcClientCallFactory}.
      *
@@ -483,49 +451,4 @@ protected Single<StreamingHttpResponse> request(final StreamingHttpRequester del
     private static GrpcStatusException toGrpcException(Throwable cause) {
         return fromThrowable(cause).asException();
     }
-
-    /**
-     * An interface to create multiple <a href="https://www.grpc.io">gRPC</a> clients sharing the
-     * same underlying transport instance.
-     * @deprecated Use {@link GrpcClientFactory#newClient(GrpcClientCallFactory)}
-     * or {@link GrpcClientFactory#newBlockingClient(GrpcClientCallFactory)}
-     * and provide a custom {@link GrpcClientCallFactory} instead.
-     */
-    @Deprecated
-    public interface MultiClientBuilder {
-
-        /**
-         * Builds a <a href="https://www.grpc.io">gRPC</a> client.
-         *
-         * @param clientFactory {@link GrpcClientFactory} to use.
-         * @param <Client> <a href="https://www.grpc.io">gRPC</a> service that any client built
-         * from this factory represents.
-         * @param <Filter> Type for client filter
-         * @param <FilterableClient> Type of filterable client.
-         * @param <FilterFactory> Type of {@link GrpcClientFilterFactory}
-         *
-         * @return A <a href="https://www.grpc.io">gRPC</a> client.
-         */
-        <Client extends GrpcClient<?>,
-                Filter extends FilterableClient, FilterableClient extends FilterableGrpcClient,
-                FilterFactory extends GrpcClientFilterFactory<Filter, FilterableClient>> Client
-        build(GrpcClientFactory<Client, ?, Filter, FilterableClient, FilterFactory> clientFactory);
-
-        /**
-         * Builds a blocking <a href="https://www.grpc.io">gRPC</a> client.
-         *
-         * @param clientFactory {@link GrpcClientFactory} to use.
-         * @param <BlockingClient> Blocking <a href="https://www.grpc.io">gRPC</a> service that
-         * any client built from this builder represents.
-         * @param <Filter> Type for client filter
-         * @param <FilterableClient> Type of filterable client.
-         * @param <FilterFactory> Type of {@link GrpcClientFilterFactory}
-         *
-         * @return A blocking <a href="https://www.grpc.io">gRPC</a> client.
-         */
-        <BlockingClient extends BlockingGrpcClient<?>,
-                Filter extends FilterableClient, FilterableClient extends FilterableGrpcClient,
-                FilterFactory extends GrpcClientFilterFactory<Filter, FilterableClient>> BlockingClient
-        buildBlocking(GrpcClientFactory<?, BlockingClient, Filter, FilterableClient, FilterFactory> clientFactory);
-    }
 }