chore: Remove unused feed

Author: jeromelaban

nuget.config

@@ -4,7 +4,6 @@
     <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
     <add key="AzureLatest" value="https://pkgs.dev.azure.com/dotnet/CommunityToolkit/_packaging/CommunityToolkit-MainLatest/nuget/v3/index.json" protocolVersion="3" />
-    <add key="MUX-Shared" value="https://pkgs.dev.azure.com/ms/microsoft-ui-xaml/_packaging/MUX-Shared/nuget/v3/index.json" />
     <add key="unoplatformdev" value="https://pkgs.dev.azure.com/uno-platform/1dd81cbd-cb35-41de-a570-b0df3571a196/_packaging/unoplatformdev/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources>