greybus: svc: remove deprecated hotplug operations

Now that firmware has made the switch to the new interface boot
sequence, we can remove support for the deprecated hotplug and
hot-unplug operations.

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>