[libomptarget][amdgpu][nfc] Remove dead code from amdgpu plugin

[libomptarget][amdgpu][nfc] Remove dead code from amdgpu plugin

Drops an enum that was identical to a HSA one, localises some functions where
they were only called from one TU. Covers everything internalize + adce can
identify as dead, except for msgpack::dump which is useful when debugging.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D102014