[clang] [flang] [llvm] [openmp] [OpenMP] Remove LLVM_ENABLE_PROJECTS=openmp build mode (PR #152189)

[Michał Górny via cfe-commits]

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

Don't see anything wrong with it. We're not using it in Gentoo, so nothing to test on my end.

https://github.com/llvm/llvm-project/pull/152189