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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Because we're on the fence about whether this has value and we're getting early feedback about diagnostic chattiness and we lack adequate test coverage for the changes, I think it's reasonable to back this out for now.

It might make more sense as a clang-tidy check should we want something along these lines.

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