MPU9K: Remove unused perf counter

src/drivers/mpu9250/mpu9250.cpp

@@ -272,7 +272,6 @@ private:
 	perf_counter_t		_good_transfers;
 	perf_counter_t		_reset_retries;
 	perf_counter_t		_duplicates;
-	perf_counter_t		_system_latency_perf;
 	perf_counter_t		_controller_latency_perf;
 
 	uint8_t			_register_wait;
@@ -533,7 +532,6 @@ MPU9250::MPU9250(int bus, const char *path_accel, const char *path_gyro, spi_dev
 	_good_transfers(perf_alloc(PC_COUNT, "mpu9250_good_transfers")),
 	_reset_retries(perf_alloc(PC_COUNT, "mpu9250_reset_retries")),
 	_duplicates(perf_alloc(PC_COUNT, "mpu9250_duplicates")),
-	_system_latency_perf(perf_alloc_once(PC_ELAPSED, "sys_latency")),
 	_controller_latency_perf(perf_alloc_once(PC_ELAPSED, "ctrl_latency")),
 	_register_wait(0),
 	_reset_wait(0),
@@ -1743,7 +1741,6 @@ MPU9250::measure()
 	if (accel_notify && !(_pub_blocked)) {
 		/* log the time of this report */
 		perf_begin(_controller_latency_perf);
-		perf_begin(_system_latency_perf);
 		/* publish it */
 		orb_publish(ORB_ID(sensor_accel), _accel_topic, &arb);
 	}