Remove dead code: ModalContainerObserver and ModalContainerControlClassifier
<rdar://problem/108871402>
Pull request: https://github.com/WebKit/WebKit/pull/13429
Committed 263667@main (65b13cfb8a8b): <https://commits.webkit.org/263667@main> Reviewed commits have been landed. Closing PR #13429 and removing active labels.