[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: 81a7cad2ffc18f15b732f69d991c8398c979c5ca
5 files changed