Remove obsolete `is_chromeos_ash` build arg

It has long been synonymous with `is_chromeos`.

File build/config/chromeos/ui_mode.gni will be deleted separately.

Bug: b:354842935
Change-Id: I27925cf3f6bd836aac2bfed5248a6f2325d0f703
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7875801
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Auto-Submit: Georg Neis <neis@chromium.org>
Owners-Override: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1639359}
NOKEYCHECK=True
GitOrigin-RevId: 2c2d89eee2c41e0ed29cb1a1d4648239a88f1e8a