[clang][PowerPC] Remove remaining Darwin support

POWER Darwin support in the backend has been removed for some time: https://discourse.llvm.org/t/rfc-remove-darwin-support-from-power-backends
but Clang still has the TargetInfo and other remnants lying around.

This patch does some cleanup and removes those and other related frontend support still remaining. We adjust any tests using the triple to either remove
the test if unneeded or switch to another Power triple.

Reviewed By: MaskRay, nemanjai

Differential Revision: https://reviews.llvm.org/D146459