[compiler-rt] [libFuzzer] Remove unused helper `GetPreviousInstructionPc` (PR #228453)

[Rong Mantle Bao via llvm-commits]

CSharperMantle wrote:

> P.S. GitHub is having some issues with merging LLVM pull requests ([discourse.llvm.org/t/psa-project-wide-issue-with-merging-pull-requests-in-the-github-ui/91974](https://discourse.llvm.org/t/psa-project-wide-issue-with-merging-pull-requests-in-the-github-ui/91974)), but rest assured I am tapping the button :-)

Never mind :) At your convenience.

https://github.com/llvm/llvm-project/pull/228453