Display categories on File redirects

Display categories on File redirects

This NS_FILE-specific redirect logic was introduced in 2008,
in 3c9536652a3b27ec97cb2e3f046c5e98849c58a4.

mTitle, as referenced to in the comments, no longer exists.
Article also no longer creates redirects in there, that’s now
in WikitextContent::fillParserOutput.
The original commit message describes that this was introduced
to be able to display redirects to foreign file repos. A lot
has changed in how that works since 2008, too :)

Back to the present: for redirects to foreign repos,
`$this->mPage->getFile()->getRedirected()` seems to always return
`null` (a placeholder object is created when the thing doesn’t
exist), so this this foreign repo-specific bit of code is bypassed
anyway. For local redirects, Article::view seems to be working just
fine.

By using Article::view, Categories used on file redirect pages
will now actually be displayed too.

And we can get rid of the otherwise no longer used Article::viewRedirect.

Bug: T29857
Change-Id: Icb02f3af32d10870f58d945cb06a84b3ba1305d3