[OpenMP] Completely remove old device runtime

This patch completely removes the old OpenMP device runtime. Previously,
the old runtime had the prefix `libomptarget-new-` and the old runtime
was simply called `libomptarget-`. This patch makes the formerly new
runtime the only runtime available. The entire project has been deleted,
and all references to the `libomptarget-new` runtime has been replaced
with `libomptarget-`.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D118934