Don't register cohost types with the Razor project system

[davidwengier]

Part of #9519

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1939874

FYI @333fred

After talking to @DustinCampbell this morning I decided to slightly change tact with cohosting, and not worry about having it be able to co-exist with the current language server. That means this RPS regression is easy to fix just by disconnecting the cohost OpenDocumentGenerator from that system. Yes, that means co-hosting is now slightly worse, but if we move away from co-existence, then it matters a lot less, as we no longer expect it to fully work, it just needs to work enough to let us all get our work done. This PR doesn't change that.

In reality what this change does is that if cohosting is on, and the old server is off, then semantic tokens won't be correct for files that were already open when you stared VS. You have to close and reopen the file. No big deal IMO

[davidwengier]

Val build, in order to generate an insertion, to verify: https://dev.azure.com/dnceng/internal/_build/results?buildId=2356357
Insertion: https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/522030

[davidwengier]

Perf DDRITs passed, so I think that means this fixes the issue.