[Cocoa] Inline an unnecessary one-line lambda within preparePlatformFont()
<rdar://problem/105438464>
Pull request: https://github.com/WebKit/WebKit/pull/10077
Committed 260248@main (b323e68196e9): <https://commits.webkit.org/260248@main> Reviewed commits have been landed. Closing PR #10077 and removing active labels.