Previous patches moved SystemPreview to use a WKDataTask. Remove the old code that was called from the LegacyDownloadClient.
<rdar://problem/108532798>
Pull request: https://github.com/WebKit/WebKit/pull/13180
Committed 263476@main (353f4a532175): <https://commits.webkit.org/263476@main> Reviewed commits have been landed. Closing PR #13180 and removing active labels.