[OpenMP] Remove use of removed '-f[no-]openmp-new-driver' flag

The changes in D130020 removed all support for the old method of
compiling OpenMP offloading programs. This means that
`-fopenmp-new-driver` has no effect and `-fno-openmp-new-driver` does
not work. This patch removes the use and documentation of this flag.
Note that the `--offload-new-driver` flag still exists for using the new
driver optionally with CUDA and HIP.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D133367