delete IOCTL SENSOR_POLLRATE_MANUAL

- only used in self tests

src/drivers/distance_sensor/srf02/srf02.cpp

@@ -344,12 +344,6 @@ SRF02::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 	case SENSORIOCSPOLLRATE: {
 			switch (arg) {
 
-			/* switching to manual polling */
-			case SENSOR_POLLRATE_MANUAL:
-				stop();
-				_measure_ticks = 0;
-				return OK;
-
 			/* zero would be bad */
 			case 0:
 				return -EINVAL;