greybus: firmware: remove skip-disconnected flag

Remove the legacy protocol flag that was used to suppress sending the
control disconnected operation.

Instead rely on the fact that no control operations will be sent by
core to an interface that has been removed (e.g. after having received a
new hotplug event after the bootrom has jumped to the new image).

Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>