purge_orphans: optimize the query, maybe

https://tt-rss.org/forum/viewtopic.php?f=1&t=3417&p=20408#p20407

include/functions.php

@@ -320,7 +320,7 @@
 
 		// purge orphaned posts in main content table
 		$result = db_query("DELETE FROM ttrss_entries WHERE
-			(SELECT COUNT(int_id) FROM ttrss_user_entries WHERE ref_id = id) = 0");
+			NOT EXISTS (SELECT ref_id FROM ttrss_user_entries WHERE ref_id = id)");
 
 		if ($do_output) {
 			$rows = db_affected_rows($result);