greybus: interface: localize gb_interface_destroy()

Its not referenced by svc or any other code anymore, lets stop exposing
it to rest of the files.
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Perry Hung <perry@leaflabs.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/interface.h

@@ -52,7 +52,6 @@ struct gb_interface *gb_interface_find(struct greybus_host_device *hd,
 struct gb_interface *gb_interface_create(struct greybus_host_device *hd,
 					 u8 interface_id);
 int gb_interface_init(struct gb_interface *intf, u8 device_id);
-void gb_interface_destroy(struct gb_interface *intf);
 void gb_interface_remove(struct greybus_host_device *hd, u8 interface_id);
 void gb_interfaces_remove(struct greybus_host_device *hd);