greybus: connection: remove legacy protocol id from core

Remove legacy protocol-id field that is only used by the legacy-protocol
driver from the connection structure.

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>