Remove Lacros leftovers from chrome/browser/lifetime

Lacros is gone.

- Replace any IS_CHROMEOS_ASH uses with IS_CHROMEOS.
- Replace any is_chromeos uses with is_chromeos.
- Remove any IS_CHROMEOS_LACROS uses and associated code.
- Remove any is_chromeos_lacros uses.
- Where appropriate, remove includes of build/chromeos_buildflags.h or
  replace with build/build_config.h.
- Remove any imports of build/config/chromeos/ui_mode.gni.

Bug: b:354842935
Change-Id: Ib2eb8a752e4dda958c63bef0ef303c17898c12d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6242356
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Di Wu <diwux@google.com>
Cr-Commit-Position: refs/heads/main@{#1419152}