[ticket/11201] Revert WLM dropping because it is still used in China.

Windows Live Messenger is still in use in china which accounts for ~20% of world
population. Revert WLM dropping which has been merged under the assumption that
WLM data and features are completely useless.

This commit reverts commits
 - 460470229d
 - 9affd6f7e7
which have been merged by d59431691c.

PHPBB3-11201

phpBB/install/database_update.php

@@ -1196,11 +1196,6 @@ function database_update_info()
 					'user_timezone'		=> array('VCHAR:100', ''),
 				),
 			),
-			'drop_columns'		=> array(
-				USERS_TABLE			=> array(
-					'user_msnm',
-				),
-			),
 		),
 	);
 }