[PGO] Remove legacy PM passes

Legacy PM for optimization pipeline was deprecated in 13.0.0 and Clang dropped
legacy PM support in D123609. This change removes legacy PM passes for PGO so
that downstream projects won't be able to use it. It seems appropriate to start
removing such "add-on" features like instrumentations, before we remove more
stuff after 15.x is branched.

I have checked many LLVM users and only ldc[1] uses the legacy PGO pass.

[1]: https://github.com/ldc-developers/ldc/issues/3961

Reviewed By: davidxl

Differential Revision: https://reviews.llvm.org/D123834