Remove support for //net/proto2/go:proto dependency

This is a follow up to bazelbuild@1a52509,
which added //net/proto2/go:protodeps as a new dependency
for every go_proto_library rule.
For the past year, we have been using protodeps exclusively
and the proto package is no longer a necessary dependency,
so we're removing support for injecting it.

RELNOTES: Generated Go protobufs no longer depend on //net/proto2/go:proto
PiperOrigin-RevId: 335538835

src/test/java/com/google/devtools/build/lib/packages/util/MockProtoSupport.java

@@ -173,9 +173,7 @@ private static void createNetProto2(MockToolsConfig config) throws IOException {
         "net/proto2/go/BUILD",
         "package(default_visibility=['//visibility:public'])",
         "go_library(name = 'protodeps',",
-        "           srcs = [ 'protodeps.go' ])",
-        "go_library(name = 'proto',",
-        "           srcs = [ 'proto.go' ])");
+        "           srcs = [ 'protodeps.go' ])");
     config.create("net/proto2/compiler/go/BUILD",
         "package(default_visibility=['//visibility:public'])",
         "go_binary(name = 'protoc-gen-go',",