[PA] Trim compiler_specific.h

PartitionAllocator's compiler_specific.h forked from Chromium's copy,
but many macros are not used. Remove them.

Change-Id: I7149427a74426916512b8134e4d1127430e6aeba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4348553


Reviewed-by: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1120043}