appDisplay: Don't add row and column spacing to the pagePadding

appDisplay: Don't add row and column spacing to the pagePadding

We use the pagePadding to adjust for the large indicators that we add on the
left and right of the appGrid.

While the original padding needs to be applied to the padding for the
indicators (as it was in super.vfunc_style_changed already), there's no
reason to add an extra row and column spacing to the pagePadding on all
sides, so remove that.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3278>