Drop checkedDowncast<>() casting function, now that downcast<>() has become equivalent (as safe).
Pull request: https://github.com/WebKit/WebKit/pull/26518
Committed 276778@main (23199acc5cb4): <https://commits.webkit.org/276778@main> Reviewed commits have been landed. Closing PR #26518 and removing active labels.
<rdar://problem/125556730>