[llvm] [NFC][CodeGen] Remove dead ParallelCG.h/.cpp API (PR #95770)

[Teresa Johnson via llvm-commits]

https://github.com/teresajohnson approved this pull request.

lgtm but please correct the summary. Looks like it is LTOBackend.cpp that has the static copy, not LTOCodeGenerator.cpp

https://github.com/llvm/llvm-project/pull/95770