[PowerPC][Future] Remove "unskipableSimplifyCode()" in PPCMIPeephole.cpp

"unskipableSimplifyCode()" was added to handle unsafe BL8_NOTOC instruction
when TOC was not completely removed. The function is not needed after confirming
TOC pointer is not used in a function that uses PC-Relative addressing.

Differential Revision: https://reviews.llvm.org/D78517