[clang][openmp][NFC] Remove arch-specific CGOpenMPRuntimeGPU files

The existing CGOpenMPRuntimeAMDGCN and CGOpenMPRuntimeNVPTX classes are
just code bloat. By removing them, the codebase gets a bit cleaner.

Reviewed By: jdoerfert, JonChesterfield, tianshilei1992

Differential Revision: https://reviews.llvm.org/D113421

GitOrigin-RevId: 737c4a2673da5829caba3e45af67d0eaa611b77a
6 files changed