Remove IS_CHROMEOS_LACROS from chrome/browser/ui/views/{download,side_panel}

Lacros is obsolete and to be removed from the Chromium code base.

R=hidehiko@chromium.org, neis@chromium.org

Bug: 377410087, 373971535
Change-Id: I0fa2b469100e8da008cb93842e4db61ee17aaf93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5992806


Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1378931}