nixos/samba: remove services.samba.defaultShare option

It's not that difficult to define shares using standard samba config
file syntax, so why do we need the semi-configurable .defaultShare
option?

Also:
 * It uses /home/smbd and I think /home should be reserved
   for real human users.
 * If enabled, it breaks the assumption that .extraConfig continues in
   the [global] section.

Without .defaultShare there is no need for the "smbguest" user and group
either, mark them as unused.