[llvm] [openmp] [offload][openmp] - Remove standalone build in favor of 'runtimes' (PR #170693)

[Shilei Tian via llvm-commits]

shiltian wrote:

I don’t think we should drop standalone builds. Many users of `libomp` don't care about offloading at all, so losing the ability to build it standalone would definitely be a step backward.

https://github.com/llvm/llvm-project/pull/170693