trail: remove excess presence-dependencies
Since trail members are explicitly rebuilt if the information used for
their prev/up/next boxes changes, they don't need another dependency
on the trail itself. (If the trail disappears, it will disappear from
the member's member_to_trails entry, causing a rebuild; so the add_depends
is redundant.)
Similarly, since trail members are explicitly rebuilt if their next
or previous item, or its title, changes, the presence dependencies on the
next and previous items are redundant.