[X86]Remove X86-specific dead code in ScheduleDAGRRList.cpp (#67629)

After patch https://github.com/llvm/llvm-project/pull/67288 landed,
unfoldMemoryOperand would not return NewMIs whose size ==3. So the
removed line is useless.

GitOrigin-RevId: fef1bec39642d7733eae22d6e79ac07c779bdef5