[Libomptarget] Remove dependency on the DeviceRTL from the GPU plugins

The GPU plugins have a dependency on the device libraries. Sometimes we
cannot build the device libraries because the user does not have a valid
`clang` to use or it was explicitly disabled. Currently this leads to a
transitive failure because we cannot meet this dependency. This patch
simply removes that dependency.

Fixes https://github.com/llvm/llvm-project/issues/60457

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D143196