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

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

llvm/utils/release/build_llvm_release.bat and clang/utils/analyzer/entrypoint.py also mention LLVM_ENABLE_PROJECTS=openmp .

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