Delete obsolete old preview link redirection

[okurz]

The redirection based on the request header type should be considered
obsolete after enough time. Also it is hard to understand why a certain
URL is redirecting in one case and behaves different when called with a
different request header.

Related progress issue: https://progress.opensuse.org/issues/72292

[Martchus]

I've been searching for /modules/.*/steps/ and it seems only tests still refer to that URL. So it should be safe to delete as long as there are no more external users.

[okurz]

hm, just found out that we still have "modules" URLs for the "src" and "edit" targets. This feels inconsistent but removing the parent URL route completely does not feel like it would help a lot:
https://github.com/os-autoinst/openQA/blob/master/t/27-errorpages.t#L43

But I don't have a good idea if we can do anything about that. Unless someone has a better idea I would leave everything as is. The redirect depending on the header is confusing but removing the route is IMHO worse.