workspaceThumbnail: Drop the _background hack

The _background hack was added because the old way the zooming animation
worked, it set the allocation of the workspaces view and thumbnails box
to the final position and used animations to smoothly animate.

During the 3.6 cycle when we added the new search view, Cosimo changed the
way the zoom animation works so that rather than set the final allocation
and animate, we actually do adjust the allocation of the workspaces view
and thumbnails box.

So, as the hack is no longer necessary, we can drop it.

https://bugzilla.gnome.org/show_bug.cgi?id=694881