greybus: interface: remove unused function prototype

Remove unused gb_interface_destroy() prototype.

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