[clang] [flang] [llvm] [openmp] [OpenMP] Remove LLVM_ENABLE_PROJECTS=openmp build mode (PR #174963)

[Michael Kruse via cfe-commits]

Meinersbur wrote:

This time it broke [publish-doxygen-docs](https://lab.llvm.org/buildbot/#/builders/111/builds/2340). I don't remember having received a notifcation of publish-doxygen-docs failing last time.

https://github.com/llvm/llvm-project/pull/174963