Remove WTF::LegacyIdentified now that it is no longer used.
Pull request: https://github.com/WebKit/WebKit/pull/25691
Committed 275900@main (ad04066591e9): <https://commits.webkit.org/275900@main> Reviewed commits have been landed. Closing PR #25691 and removing active labels.
<rdar://problem/124389553>