appDisplay: Remove unnecessary bind constraint

appDisplay: Remove unnecessary bind constraint

Now that the bind constraint changes the preferred size of
the actor, a major flaw in the AppDisplay code was exposed:
the folder dialog depends on the preferred size of a parent,
and the parent depends on the preferred size of the folder
dialog.

While we know this is not actually true, we shouldn't rely
on broken behavior to achieve this result. What's interesting
is that the bind constraint used by the folder dialog is a
relic of the development phase; we now control the position
and size of the dialog with a combination of CSS, and alignment.

Remove the unnecessary bind constraint.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1027