Reland "Cleanup checkout_chromeos_assistant var"

This is a reland of commit ac4b6ee9

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: Ice1f1053ad82d4bb9b23d12a0e579fbb0687fd4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4375421


Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>
Cr-Commit-Position: refs/heads/main@{#1123202}