Move wallpapers to recommends and remove gsettings override

[jacobgkau]

The gsettings override will be added to the system76-wallpapers package instead. This will allow users to remove the system76-wallpapers package (or install the driver with --no-recommends) if they don't want it.

[jacobgkau]

Testing this along with https://github.com/pop-os/system76-wallpapers/pull/2:

• On a fresh Ubuntu 20.04 LTS installation, installing system76-driver still installs and sets the default System76 wallpaper (takes effect on next shell restart.)
  • After that, the package can be removed with sudo apt remove system76-wallpapers, which resets the background to GNOME's default blue.
• On a fresh Ubuntu 20.04 LTS installation, installing system76-driver with --no-install-recommends leaves the default Ubuntu wallpaper in place, even after a shell restart.
  • After that, installing system76-wallpapers still switches the wallpaper on the next shell restart.
• Installing the updates on Pop!_OS doesn't affect anything (pop-default-settings still uses a gsettings override priority of 50 while system76-wallpapers keeps the priority of 40 that system76-driver was using.)

Seems to be working with no ill side-effects.