remove unneeded load_pref for thread sort, fixes warning with E_ALL

src/load_prefs.php

@@ -217,8 +217,7 @@ $sig_first = getPref($data_dir, $username, 'sig_first', 0);
 /* use the internal date of the message for sorting instead of the supplied header date */
 $internal_date_sort = getPref($data_dir, $username, 'internal_date_sort', SMPREF_ON);
 
-/* if thread sorting is enabled/disabled */
-$thread_sort_messages = getPref($data_dir, $username, "thread_$mailbox", 0);
+/* if server sorting is enabled/disabled */
 $sort_by_ref = getPref($data_dir, $username, 'sort_by_ref', 1);
 
 /* Load the javascript settings. */