[libFuzzer] Remove unused helper `GetPreviousInstructionPc` (#228453)

This function hasn't been used since 2891b257c24c ("[libFuzzer] remove
stale code"), and it contains a typo in the RISCV #ifdef arm. Removing
this altogether.

This was found when surveying RISCV-specific code in [a downstream
project](https://searchfox.org/firefox-main/rev/084057e952e7dbf376f6c3765ad242aec3785dc6/tools/fuzzing/libfuzzer/FuzzerTracePC.cpp#141)
that vendored libFuzzer.

GitOrigin-RevId: 120e630a18f842bccfea855dae30d7279aa33df5
1 file changed