partition_alloc: merge buildflags.h #cleanup

partition_alloc has 5 different buildflags headers, but there isn't a
need to maintain multiple:
- All of them were anyway grouped under the "buildflags" target.
- All of them are included in every files via the `raw_ptr.h` file.

To keep things simpler to maintain, this merges them all.

This is #cleanup.

Why No-Try: It seems touching some PA files causes the CQ to run too
many tests and they are slower on the CQ due to option gathering code
coverage. As an example, adding empty lines causes the same failure:
https://chromium-review.googlesource.com/c/chromium/src/+/5513777
Feel free to revert this patch if some a bots fails two consecutive
run.

No-try: true
Bug: 41481467
Change-Id: I56a30e5bb453c19a8d262f7a19b3d76ecae23a26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5630428
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Owners-Override: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1319627}
NOKEYCHECK=True
GitOrigin-RevId: fd39d619c7eadd934eb46f2d9d05821e71c9d491
1 file changed