Delete ppapi/buildflags/buildflags.h

Transition ENABLE_PLUGINS users to content/public/common/buildflags.h.
Then delete ppapi/buildflags/buildflags.h and its GN target.

Bug: 40511450
Change-Id: I97d19a2a8a20c24ab95d0aec91f9903b6d5c8088
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6690442
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Owners-Override: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1487200}
NOKEYCHECK=True
GitOrigin-RevId: ae572bef3647189819f1371c5da4841b25afa124