Clean lacros-specific code in chrome/browser/ui/chromeos (1/n)

This CL deletes the lacros-only code in c/b/ui/chromeos/
directory, now that the project has been terminated.

Particularly, the CL does:
- deletion of code guarded by BUILDFLAG(IS_CHROMEOS_LACROS).
- deletion of the remaining BUILDFLAG(IS_CHROMEOS_ASH) guards
  but not its block code inside.
- added relevant TODOs.
- removal of inclusions of `build/chromeos_buildflags.h`.

R=hidehiko@chromium.org

Bug: 374896328, 373971535
Change-Id: Ieca0bf26806b4e7ee60b60c44f696dc35cda1f54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5953385
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1372340}