Remove richnav.yml and feed (#61209)

The service no longer exists.

Author: akoeplinger

.azure/pipelines/richnav.yml

@@ -11,8 +11,6 @@
     <add key="dotnet8" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json" />
     <add key="dotnet8-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8-transport/nuget/v3/index.json" />
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
-    <!-- Used for the 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 />