Feed cleanup (#42739)

* Feed cleanup

Remove blob feeds (and some other feeds I don't think are required)

NuGet.config

@@ -3,16 +3,11 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="dotnet-core" value="https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json" />
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="dotnet-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json" />
     <add key="dotnet5" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet5/nuget/v3/index.json" />
-    <add key="system-commandline" value="https://dotnet.myget.org/F/system-commandline/api/v3/index.json" />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
-    <add key="myget-dotnet-coreclr" value="https://dotnet.myget.org/F/dotnet-coreclr/api/v3/index.json" />
-    <add key="myget-dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
     <add key="dotnet-corefxtestdata" value="https://dotnet.myget.org/F/dotnet-corefxtestdata/api/v3/index.json" />
-    <add key="dotnet-buildtools" value="https://dotnet.myget.org/F/dotnet-buildtools/api/v3/index.json" />
     <add key="experimental-vs-packages" value="https://dotnet.myget.org/F/experimental-vs-packages/api/v3/index.json" />
     <add key="symreader-native" value="https://dotnet.myget.org/F/symreader-native/api/v3/index.json" />
     <add key="interactive-window" value="https://dotnet.myget.org/F/interactive-window/api/v3/index.json" />
@@ -25,9 +20,6 @@
     <add key="roslyn_concord" value="https://myget.org/F/roslyn_concord/api/v3/index.json" />
     <add key="vssdk" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vssdk/nuget/v3/index.json" />
     <add key="vs-impl" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-impl/nuget/v3/index.json" />
-    <add key="system-commandline" value="https://dotnet.myget.org/F/system-commandline/api/v3/index.json" />
-    <add key="dotnet-coreclr" value="https://dotnetfeed.blob.core.windows.net/dotnet-coreclr/index.json" />
-    <add key="dotnet-format" value="https://dotnet.myget.org/F/format/api/v3/index.json" />
   </packageSources>
   <disabledPackageSources>
     <clear />