Cleanup nuget feeds (#1928)

NuGet.config

@@ -2,7 +2,8 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="dotnet-core" value="https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json" />
+    <add key="dotnet3" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet3/nuget/v3/index.json" />
+    <add key="dotnet3-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet3-transport/nuget/v3/index.json" />
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>