Exo: Remove is_chromeos_ash from components/exo BUILD.gn

Exo is no longer used except for ash.

Bug: 1342970
Change-Id: Ia436c6b87eb8caeb4a3628d0e64fb2992dd62442
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4060491


Commit-Queue: Eriko Kurimoto <elkurin@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1076566}