greybus: Documentation: remove svc unique_id attribute

Remove unimplemented svc unique_id attribute from the documentation.

This attribute made more sense when we thought we'd have an AP-module,
unlike now when the AP and SVC are both part of the same frame.

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