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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> …and that would be because all the logic for finding installed `openmp` library/headers is removed.

That'd do it, I suppose we still want to use `DetectTestCompiler` in this case? It could be either in a default runtimes build.

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