Revert change that added preview2-final NuGet feed as it's not required in dev branch

Author: SteveSandersonMS

NuGet.config

@@ -3,7 +3,6 @@
   <packageSources>
     <clear />
     <add key="AspNetCore" value="https://dotnet.myget.org/F/aspnetcore-ci-dev/api/v3/index.json" />
-    <add key="AspNetCorePreview2Final" value="https://dotnet.myget.org/F/dotnet-2-0-0-preview2-final/api/v3/index.json" />
     <add key="AspNetCoreTools" value="https://dotnet.myget.org/F/aspnetcore-tools/api/v3/index.json" />
     <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
   </packageSources>