Unset the scroll_to attribute to stop the panning timeout properly.

* hildon/hildon-pannable-area.c,
(hildon_pannable_axis_scroll),
(hildon_pannable_area_scroll): Unset the scroll_to in some
situations, it was causing the timeout did not stop in some cases.

Fixes: NB#140186 (HildonPannableArea goes into an infinite loop)