[AMDGPU][Libomptarget] Remove g_atmi_machine global

Turns out the only purpose of this class was verify if device ID
was in range or not which could be done easily by using g_atl_machine.

Still getting rid of g_atl_machine is pending which would be done in
a later patch.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D103443