Simplify JSStringJoiner by dropping one of its constructors and one of its data members.
Pull request: https://github.com/WebKit/WebKit/pull/11971
Committed 262115@main (e5ebb31dd3b9): <https://commits.webkit.org/262115@main> Reviewed commits have been landed. Closing PR #11971 and removing active labels.
<rdar://problem/107226085>