[clang] [flang] [flang][OpenMP] Remove experimental warning (PR #144915)

[Tom Eccles via cfe-commits]

tblah wrote:

[ebd2980](https://github.com/llvm/llvm-project/pull/144915/commits/ebd2980051b3994d1c419d06f3cba22927283aec) moves the warning to the frontend driver so it can use the checks added in #145725

https://github.com/llvm/llvm-project/pull/144915