We don't need this flag anymore, the feature is maturing fast and there is a runtime flag for it as well.
Pull request: https://github.com/WebKit/WebKit/pull/5649
Committed 255862@main (6ac3761da1f2): <https://commits.webkit.org/255862@main> Reviewed commits have been landed. Closing PR #5649 and removing active labels.
<rdar://problem/101453579>