greybus: svc: drop legacy-protocol dependency

Drop dependency on the legacy protocol abstraction.

Remove the now unused and last legacy-protocol flag
GB_PROTOCOL_SKIP_VERSION along with the protocol-flag feature.

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>