This reverts commit 359eba13.

What's been done:
- All the RTM references have not been changed
- All the prerelease refs have been bumped
- All the projects with prerelease refs have been
  changed to use netstandard1.3 instead of dotnet5.4
- Crossgen has been updated to point to the new CLR directory
- The NuSpecs have been updated to use the new refs

There is also now a stand-alone project for referencing
CoreCLR that can be updated independently of corefx. It will
need to be added to the closed solution before the refs in
the existing projects can be removed.

This reverts commit 760e1204.

This reverts commit e2fc5135.

Synchronize all corefx references to -23504 and bring in the
xunit coreclr test runner.

These supports sections (as best I can tell) should be here, but since
we aren't including a reference to Microsoft.NETCore.Targets, the
supports targets are unknown and thus just giving warnings. We should
revert this and fix the underlying issue, but after a half hour I
only have more questions than answers. So this isn't great, but it's
better than not doing this.