native-window-placement: Remove custom styling

The window-picker padding was causing it to become smaller in the
overview resulting in a jump when opening it and caused sizing issues
with the workspace view in the app picker. However it is not needed
anymore with the new overview, so this can be fixed by simply removing
it.

The horizontal- and vertical-spacing properties got replaced with a
spacing property a while ago. However this is only used in
WorkspaceLayout::_createBestLayout() which gets overridden by this
extension which does not use it. So they can simply be removed.

The shell-caption-spacing property got removed when the window captions
got changed to always use the full length and has not been doing
anything since.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/301
Fixes: https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/309
(cherry picked from commit 4a26cecd)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/202>