[ticket/13454] Remove unused variables

This is part 2 of the pr.

PHPBB3-13454

phpBB/includes/functions_transfer.php

@@ -406,9 +406,6 @@ class ftp extends transfer
 	*/
 	function _put($from_file, $to_file)
 	{
-		// get the file extension
-		$file_extension = strtolower(substr(strrchr($to_file, '.'), 1));
-
 		// We only use the BINARY file mode to cicumvent rewrite actions from ftp server (mostly linefeeds being replaced)
 		$mode = FTP_BINARY;