Revert "overview: Ignore dragEnd while the animation is still in progress"

This reverts commit 80a3bb85aa4a66ed55b3d7465500e1c1f7cade6c.

https://bugzilla.gnome.org/show_bug.cgi?id=708887
layout: Flush region update in showOverview

We cannot wait for the queued update region to fire when
xdnd is being used because a wrong input shape can result
into a xdnd leave event when the user moves the pointer fast.

https://bugzilla.gnome.org/show_bug.cgi?id=708887