don't fake a 404 for non-ajax requests

[RobertHeim]

When symfony2 is updated to at least 2.4 ( which will take until phpbb 3.2 ( https://www.phpbb.com/community/viewtopic.php?f=461&t=2300491 ) we should use routing-conditions

like this: http://stackoverflow.com/a/21661883/386201 here: https://github.com/RobertHeim/phpbb-ext-topictags/blob/6152e939557b86c01fa40e0d5180faae320a595b/controller/main.php#L295-L296

instead of faking a 404 here: https://github.com/RobertHeim/phpbb-ext-topictags/blob/6152e939557b86c01fa40e0d5180faae320a595b/controller/main.php#L307