nixpkgs: apparmor - remove dbus dependency

This was untested and didn't function without a dbus patch which wasn't
applied to the system dbus package, so it wasn't used at all.

Also, it creates a weird cyclic dependency if we want systemd to depend
on libapparmor (for AppArmorProfiles= support), because libapparmor then
wants dbus, and dbus wants systemd. Oof.

Luckily, this feature and whatnot will probably all be irrelevant in the
glorious kdbus-based future, and the dbus patches aren't even upstream I
think. So we can just drop it.

Signed-off-by: Austin Seipp <aseipp@pobox.com>