greybus: Documentation/sysfs: remove interface unique_id attribute

Remove the interface unique_id attribute, which there is currently no
plan to ever implement.

Note that the Ara serial numbers are already exposed through the
serial_number attribute.
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>