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

[Thurston Dang via llvm-commits]

thurstond wrote:

> Per [RISC-V C API specs](https://github.com/riscv-non-isa/riscv-c-api-doc/blob/25091956af52dee37fc22c9f14da52598412cc48/src/c-api.adoc#preprocessor-definitions), the correct macro name for detecting the RISCV target is `__riscv`, not `__riscv__` as in the current code.
> 
> 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.
> 
> **v2** ([205825b](https://github.com/llvm/llvm-project/commit/205825bd84b35b3570532c81e25d87dc82772b7b)):
> 
> * Remove the function altogether. See [[libFuzzer] Remove unused helper `GetPreviousInstructionPc` #228453 (comment)](https://github.com/llvm/llvm-project/pull/228453#discussion_r4167796886).

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