power: Remove unused gpm_device_kind_to_icon()
power: Remove gpm_device_technology_to_localised_string()

It was used in the translated description for the device, which we
were only using when on UPS power.
power: Remove unused gpm_upower_get_device_description()
power: Device* signals from UPower use object paths

Not strings. The dbus-glib version of libupower-glib accepted
both happily, but the GDBus version really wants an object path,
not a string.