Remove DeskExtension that supported Lacros

Lacros is deprecated. Remove the DeskExtension and overrides in
ozone/wayland. It was decided to do this as a separate patch so
that it'd be possible to easily revert part of that in the future
if we're going to use virtual desktops in the future.
(This was discussed with nickdiego@, who is actively working on
Ozone/Wayland)

Bug: 374244479, 373971535
Change-Id: Iec913ff8dbc5abd779da7ffa25c3aa3dea9b2f73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6018323
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1382846}