[clang] [clang] Remove diagnostic that came with `[[clang::preferred_type]]` (PR #70632)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > Why doesn't this result in any changes to tests? I would expect having to remove them, right?
> 
> There was a single test that was relying on sign mismatch, and then during code review we made the diagnostic to ignore signness. So it ended up merged without test case where this diagnostic was triggered.

Ooof, that is disappointing.  We should have never let this in without test coverage.

https://github.com/llvm/llvm-project/pull/70632