greybus: interface: remove unique id

Remove the unimplemented interface unique-id.

There will eventually be an interface-serial-number attribute provided,
but let's not export it or commit to a name for this attribute until we
need it.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>