[feature/auth-refactor] Removed no longer used variable

PHPBB3-9734

phpBB/includes/acp/acp_board.php

@@ -522,7 +522,6 @@ class acp_board
 		if ($mode == 'auth')
 		{
 			// Retrieve a list of auth plugins and check their config values
-			$auth_plugins = array();
 			$auth_providers = $phpbb_container->get('auth.provider_collection');
 
 			$updated_auth_settings = false;