[Cocoa] Delete WebKitPrivateFrameworkStubs for operating systems we don't support any more
<rdar://problem/110074446>
Pull request: https://github.com/WebKit/WebKit/pull/14542
Committed 264756@main (2b73425793c0): <https://commits.webkit.org/264756@main> Reviewed commits have been landed. Closing PR #14542 and removing active labels.