[libomptarget][AMDGPU] Remove MaxParallelLevel

Removes MaxParallelLevel references from rtl.cpp and drops
resulting dead code.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D92463

GitOrigin-RevId: afc09c6fe44ecf99e5946b7fe08013f592504448
1 file changed