overviewControls: Return zero preferred size

overviewControls: Return zero preferred size

The OverviewControls actor gets allocated a fixed size by its parent,
the OverviewActor, anyway, so it's pretty useless to go through the size
request machinery and add up all the sizes of items in the iconGrid,
coming up with a preferred size that's wrong anyway.

Instead simply return a min and preferred size of 0 in
get_preferred_height/width of ControlsManagerLayout.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1755>