[LegacyPM] Remove GCOVProfilerLegacyPass

Using the legacy PM for the optimization pipeline was deprecated in 13.0.0.
Following recent changes to remove non-core features of the legacy
PM/optimization pipeline, remove GCOVProfilerLegacyPass.

I have checked many LLVM users and only llvm-hs[1] uses the legacy gcov pass.

[1]: https://github.com/llvm-hs/llvm-hs/issues/392

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D123829