Dirty roots are never shadow roots, since we use the flat tree for
invalidation. The flat tree does not contain shadow roots.
Reverts https://github.com/servo/servo/pull/35132. When that patch was
submitted we used the DOM tree for invalidation, so it was possible to
encounter shadow roots.
Testing: Covered by
`shadow-dom/untriaged/events/event-dispatch/test-003.html`
Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>