When an SVGElement is removed from a <use> element's shadow tree, we
need to check if it was in a use element's shadow root to avoid
notifying use elements about the removal of their own clones.
The check was incorrectly using root() instead of old_root. Since the
element has already been detached when removed_from() is called,
root() no longer returns the shadow root, causing the early-return
check to fail.
This led to O(n) recursion depth when clearing a use element's shadow
tree, as each removed clone would trigger another round of
remove_all_children() on use elements referencing the same ID.
