[feature/prune-users] Rework user_delete() functions_user.php

user_delete now uses fewer queries to delete a set of users of size > 1 by
accepting an array of users rather than a single user at a time.  This required
changing the third parameter, however the function retains its former behavior
with the old-style parameters.

PHPBB3-9622

phpBB/includes/acp/acp_prune.php

@@ -265,10 +265,7 @@ class acp_prune
 						}
 						else
 						{
-							foreach ($user_ids as $user_id)
-							{
-								user_delete('retain', $user_id, $usernames[$user_id]);
-							}
+							user_delete('retain', $user_ids, true);
 
 							$l_log = 'LOG_PRUNE_USER_DEL_ANON';
 						}