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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> In Gentoo, we're getting away with:
> 
> ```
> runtimes openmp cmake llvm/{cmake,include,utils/llvm-lit}
> ```

There is also third-party/ which is used for omptest since #147381.

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