greybus: drop a now-empty structure

One structure, gb_gpio_activate_response, was not deleted even
though it now has no contents.  Delete it.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/gpio-gb.c

@@ -67,8 +67,7 @@ struct gb_gpio_line_count_response {
 struct gb_gpio_activate_request {
 	__u8	which;
 };
-struct gb_gpio_activate_response {
-};
+/* activate response has no payload */
 
 struct gb_gpio_deactivate_request {
 	__u8	which;