Allow logged-in users to view and use the login form again

The fix for bug 15484 (d0439af8) has introduced the behavior of
automatically redirecting users who view the login form when already
logged in to the requested 'returnto' location. If it was not given,
users were redirected to main page instead.

However, that has annoyed people who often switch between several
accounts and have grown accustomed to the old behavior of being able
to log in while logged in. Given that there are no conflicts between
these two features, let's just restore the old behavior when
Special:UserLogin is visited directly (no 'returnto' location given).

This reverts 5dfc57eb which removed then-dead code for showing login
form to logged-in users and tweaks an if() condition.

Bug: 70855
Change-Id: I7e40c13a6ca566b4d66d943c006af9edb6941ee9
(cherry picked from commit a94f11d625ea70f253e0d9620a44b447dd830b6b)
Merge "Allow logged-in users to view and use the login form again" into REL1_24