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

[Michael Kruse via cfe-commits]

Meinersbur wrote:

> What I meant is, `libomp` was not initially a LLVM project since _its_ own day 1.

How is this relevant? libc++was also bootstrapped before it became an LLVM subproject. And even after lived in a separate SVN repository.

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