Remove WebRTC third party usrsctp
<rdar://problem/100840106>
Pull request: https://github.com/WebKit/WebKit/pull/5070
Committed 255338@main (847891fb42fa): <https://commits.webkit.org/255338@main> Reviewed commits have been landed. Closing PR #5070 and removing active labels.
Re-opened since this is blocked by bug 246873
Pull request: https://github.com/WebKit/WebKit/pull/6584
Committed 256830@main (ab5451e52d4d): <https://commits.webkit.org/256830@main> Reviewed commits have been landed. Closing PR #6584 and removing active labels.