Remove the postmarketos-base-ui-gnome-wallpapers subpackage that only
Phosh and GNOME desktop used to depend on, and instead add a hard
dependency on postmarketos-artwork-wallpapers.

The config that enables the wallpapers will be moved to
postmarketos-artwork-wallpapers in the next commit. This will make it
easier to maintain, the paths to the wallpapers will only be in one
package instead of being scattered across many.