Reapply #152189 and #174963 which were reverted because it broke
publish-sphinx-docs and publish-doxygen-docs.
The build mode has been deprecated in #136314 and was supposed to be
removed in the LLVM 21 release (#136314).
OpenMP currently supports 4 build modes:
* `cmake <llvm-project>/llvm -DLLVM_ENABLE_PROJECTS=openmp`
* `cmake <llvm-project>/llvm -DLLVM_ENABLE_RUNTIMES=openmp` (bootstrapping build)
* `cmake <llvm-project>/openmp` (standalone build)
* `cmake <llvm-project>/runtimes -DLLVM_ENABLE_RUNTIMES=openmp` (runtimes default/standalone build)
Each build mode increased the maintanance overhead since all build modes
must continue working and user confusion when there do not (see #151117,
#174126, #154117, ...). Let's finally remove it.
c15853ec3a