Fixup feeds (#2742)

Co-authored-by: Matt Mitchell <mmitche@microsoft.com>

Author: dotnet-bot

NuGet.config

@@ -6,8 +6,6 @@
   <packageSources>
     <clear />
     <!--To inherit the global NuGet package sources remove the <clear/> line below -->
-    <add key="dotnet-core" value="https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json" />
-    <add key="api.nuget.org" value="https://api.nuget.org/v3/index.json" />
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="test-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/test-tools/nuget/v3/index.json" />
     <add key="dotnet3.1" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet3.1/nuget/v3/index.json" />