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

[Shilei Tian via cfe-commits]

shiltian wrote:

As long as we keep support standalone build for `libomp`, I'm fine with it.

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