[ticket/14264] Don't use constants as return values

This will prevent BC breaking in the future if we decide to
get rid of constants.

PHPBB3-14264

phpBB/phpbb/textreparser/plugins/user_signature.php

@@ -54,14 +54,6 @@ class user_signature extends \phpbb\textreparser\row_based_plugin
 		);
 	}
 
-	/**
-	* {@inheritdoc}
-	*/
-	public function get_table_name()
-	{
-		return USERS_TABLE;
-	}
-
 	/**
 	* Save the keyoptions var from \phpbb\user
 	*/