greybus: greybus_protocols: remove svc key-event type

Remove the deprecated svc key-event type, which has already been removed
from the specification.

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>