[Build] Fix nuget.config (#13714)

* [Build] Fix nuget feed

* [Build] Remove not used nuget.config

Author: rmarinho

DevopsNuget.config

@@ -2,9 +2,9 @@
 <configuration>
   <packageSources>
     <clear/>
-    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" protocolVersion="3" />
-    <add key="nightly" value="https://aka.ms/xf-ci/index.json" />
+    <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" protocolVersion="3" />
+    <add key="dotnet6" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet6/nuget/v3/index.json" />
     <add key="nuget" value="../s/Nuget" />
   </packageSources>
   <activePackageSource>