[ticket/13454] Remove unused variables

This is part 5 and there is more to come.

PHPBB3-13454

phpBB/includes/sphinxapi.php

@@ -1659,7 +1659,6 @@ class SphinxClient
 			return false;
 		}
 
-		$res = substr ( $response, 4 ); // just ignore length, error handling, etc
 		$p = 0;
 		list ( $rows, $cols ) = array_values ( unpack ( "N*N*", substr ( $response, $p, 8 ) ) ); $p += 8;