Remove the .NET Core Tools NuGet source

It's not needed anymore now that Microsoft.DotNet.ApiCompat is replaced with the builtin package validation from the .NET 6 SDK (see commit 7133cce).

Author: 0xced

NuGet.config

@@ -3,7 +3,5 @@
   <packageSources>
     <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
-    <!-- For Microsoft.DotNet.ApiCompat -->
-    <add key=".NET Core Tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
   </packageSources>
 </configuration>