Cleanup //ui/gfx from LaCros support code.

No functional changes are done.

1) Removed code under IS_CHROMEOS_LACROS
2) Renamed IS_CHROMEOS_ASH to IS_CHROMEOS
3) Removed chromeos_buildflags.h includes and deps
4) General clean ups.

PS had to also rename IS_CHROMEOS_ASH to IS_CHROMEOS in
//chrome/browser/ui/webui/settings/search_engines_handler.h
as it implicitly was including chromeos_buildflags.h via //ui/gfx.

Bug: 373972275, 373971535, 377325000
Change-Id: I355c221c56d1d23433110ee355a6380a4903d8ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5993496
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1381067}