[code health] Remove dead case from LayoutSVGTransformableContainer

As of https://chromium-review.googlesource.com/c/chromium/src/+/1986325
we no longer replace nested <use/> elements with <g> elements when
populating shadow trees and instead support <use/> inside shadow trees
directly (as per the current spec).  Remove the check for this case
from LayoutSVGTransformableContainer as it has been dead for a while.

Change-Id: Iad3a09fe4779cdd54edd0ae98aaaac07fc1bcfb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462227


Commit-Queue: George Steel <gtsteel@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816661}