scrolledwindow: Remove overshoot window and displacement animation

scrolledwindow: Remove overshoot window and displacement animation

The displacement animation has been replaced by edge gradients, that
have a stronger color the harder overshooting is hit. This makes it
possible to remove the internal overshoot window, which was merely
used to have contents displaced when overshooting to top/left.

Overshooting to bottom/right used to cause queue_resize() to be
called on the scrolled window, this isn't necessary anymore either.

https://bugzilla.gnome.org/show_bug.cgi?id=731297