Remove corefxlab as a package source

Author: AArnott

src/nuget.config

@@ -6,7 +6,6 @@
   <packageSources>
     <add key="api.nuget.org" value="https://api.nuget.org/v3/index.json" />
     <add key="myget.org/F/aarnott" value="https://www.myget.org/F/aarnott/api/v3/index.json" />
-    <add key="corefxlab" value="https://dotnet.myget.org/F/dotnet-corefxlab/api/v3/index.json" />
     <add key="Nerdbank.MSBuildExtension CI" value="https://ci.appveyor.com/nuget/nerdbank-msbuildextension" />
   </packageSources>
 </configuration>