Removed nuget from the default build configuration

Nuget packages now have to be built manually, either by
right-clicking OpenTK.Rift.Nuget and selecting Build,
or by invoking msbuild on
OpenTK.Rift/Nuget/OpenTK.Rift.Nuget.csproj.

This simplifies builds for most users, who are not
interested in publishing new OpenTK.Rift nuget packages.