Remove redundant preview2 NuGet feed reference

Author: SteveSandersonMS

NuGet.config

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