[llvm] [Casting][NFC] Remove soft deprecation on functions with `_or_null` suffix (PR #218876)

[Yanzuo Liu via llvm-commits]

zwuis wrote:

I read the whole thread before submitting this PR. I don't have a strong preference on this matter.

>From the discussion, I gathered that most people want to keep the `_or_null` suffix, so I think removing the soft deprecation is reasonable.

https://github.com/llvm/llvm-project/pull/218876