[NFC]][PowerPC] Remove unused intrinsic for old CTR loop pass

Summary:

In the patch D62907 the PPC CTRLoops pass has been replaced by Generic
Hardware Loop pass, and it has imported some new intrinsic for Generic
Hardware Loop.

The old intrinsic used in PPC CTRLoops int_ppc_mtctr and
int_ppc_is_decremented_ctr_nonzero is been replaced by
int_set_loop_iterations and loop_decrement.

This patch is to remove above unused two instrinsic.

Reviewed By: shchenz

Differential Revision: https://reviews.llvm.org/D81539