Remove cohosting feature flag checks and associated code

[davidwengier]

In 18.3 cohosting defaults to on, in 18.4 we're removing the feature flag, so this has to go first as it checks the feature flag.

Fixes dotnet/razor#12655
Fixes #51678

I manually tested and hot reload still works, diagnostics still get reported etc. I didn't test a rude edit, because I'm not sure what one is any more, that can be achieved in a Razor file.

[jasonmalinowski]

[tmat]

what one is any more, that can be achieved in a Razor file.

Changing visibility would do:

@{
public class C {}
}

@{
internal class C {}
}

[davidwengier]

Changing visibility would do:

Thanks. Looks like this is broken, but it's broken in main too so logged dotnet/razor#12661 to investigate separately