float: Remove unnecessary logic

As `AddChild(*)` will trigger the window's workspace layout changes,
the work will be done inside
`FloatController::OnWillRemoveWindowFromLayout` is not needed to be
done inside `FloatController::UnfloatImpl` now.

See comments at crrev.com/c/4851136.

Test: existing tests
Bug: None
Change-Id: Id3a1ea15087275125b4239729a941dd629c5c688
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5530557
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1299379}