Remove dotnet-public feed after adding it as an upstream of perfview-build.

Author: brianrob

Nuget.config

@@ -5,7 +5,6 @@
   </fallbackPackageFolders>
   <packageSources>
     <clear/>
-    <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
     <add key="perfview-build" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/perfview-build/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources>