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

https://github.com/llvm/llvm-project/pull/69104 introduce a diagnostic that checked underlying type of an enum against type of bit-field that is annotated with `[[clang::preferred_type]]`. When I tried to introduce this annotation in https://github.com/llvm/llvm-project/pull/70349, it turned out to be too chatty, despite effort to avoid that.