Cleanup checkout_chromeos_assistant var

checkout_chromeos_assistant var was added to make libassistant migration from src-internal to src easier. This change cleans up this var since it is no longer used.

Bug: 1409738
Change-Id: I1cb3f7cb173abbfe6686b8972d37751f3267ece6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4347742


Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>
Auto-Submit: Aravind Vasudevan <aravindvasudev@google.com>
Cr-Commit-Position: refs/heads/main@{#1118505}