workspace: Remove zooming of window previews

Using the scroll wheel to switch workspaces makes sense, but it is
currently only supported on the workspace switcher, as it conflicts
with window zooming in the picker.
As changing workspaces is far more useful than the zoom feature,
remove the latter in order to "free" the scroll wheel for workspace
switching.

https://bugzilla.gnome.org/show_bug.cgi?id=686639