Revert r10106 - Index usage is very unlikely because of WHERE IN (). :-|

git-svn-id: file:///svn/phpbb/branches/phpBB-3_0_0@10107 89ea8834-ac86-4346-8a33-228a782c2dd0

phpBB/install/database_update.php

@@ -877,9 +877,6 @@ function database_update_info()
 				POSTS_TABLE			=> array(
 					'post_username'		=> array('post_username'),
 				),
-				USERS_TABLE			=> array(
-					'user_regdate'	=> array('user_regdate'),
-				),
 			),
 		),
 	);