Revert "Performance breakthrough"

This reverts commit c291685654.

include/functions.php

@@ -320,7 +320,8 @@
 
 		// purge orphaned posts in main content table
 		$result = db_query("DELETE FROM ttrss_entries WHERE
-			id NOT IN (SELECT ref_id FROM ttrss_user_entries WHERE ref_id IS NOT NULL)");
+			(SELECT COUNT(int_id) FROM ttrss_user_entries WHERE ref_id = id) = 0");
+
 		if ($do_output) {
 			$rows = db_affected_rows($result);
 			_debug("Purged $rows orphaned posts.");