[flang-rt] Remove experiemental OpenMP offloading support (#183653)
Summary:
This, as far as I am aware, has mostly been superceded by the runtimes
build that's built on top of libc. This build links 30% faster, supports
more functionality, and uses 95% less disk space, so it seems to be the
direction we want to go.
CUDA support remains, this is not needed urgently.