st-scroll-view: Unconditionally allocate scrollbars

Commit cfecd063 changed the allocation logic to not allocate
scrollbars when the *_visible booleans are false. This breaks the
fade effect as well as the NEVER policy. We do not paint scrollbars
when they are not supposed to be visible, so not allocating them
and thus leaving them in a "needs allocation" state just causes problems.

I am not convinced that it solved any problem to begin with (we don't paint
them anyway).

As the previous condition has basically always been true, just do it
unconditionally.

https://bugzilla.gnome.org/show_bug.cgi?id=705664