dotnet sln remove support for slnx

[marcpopMSFT]

We're trying out the GH project support and issue parents with progress tracking. You can't parent PRs so creating an issue to track this work on our project board.

I won't bother with add, list, or migrate for the moment though I may later to backfill the progress.

#45160

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.