It's dead code. We don't support any platforms without -setThumbnailURLs:
<rdar://problem/111149527>
Pull request: https://github.com/WebKit/WebKit/pull/15201
Committed 265422@main (b8427a9866cd): <https://commits.webkit.org/265422@main> Reviewed commits have been landed. Closing PR #15201 and removing active labels.