Revert "Cleanup checkout_chromeos_assistant var"

This reverts commit ac4b6ee9.

Reason for revert: This breaks canary release crbug/1425561

Original change's description:
> 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}

Bug: 1409738
Change-Id: Ie70a74d5750641c15261a938c1bbb7355af93461
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4351047


Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118867}