Remove richnav pipeline and feed (#259)

The service no longer exists.

Author: akoeplinger

NuGet.config

@@ -9,8 +9,6 @@
     <!-- Required for Microsoft.VisualStudio.Platform.VSEditor package -->
     <add key="vs-impl" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-impl/nuget/v3/index.json" />
     <add key="vs-impl-archived" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-impl-archived/nuget/v3/index.json" />
-    <!-- Used for Rich Navigation indexing task -->
-    <add key="richnav" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-buildservices/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources>
     <clear />