ArduCopter: Revert "Switched to Gyro only startup"

This reverts commit f932f0a9f6a29e9c3193622109298c657ed5a908.

I fixed this bug in 734185599cd9d6a487c675460c09895172e012b0.

It is no longer valid to call IMU::init_gyro() before IMU::init().

ArduCopter/system.pde

@@ -385,7 +385,7 @@ static void startup_ground(void)
 	#if HIL_MODE != HIL_MODE_ATTITUDE
 		// Warm up and read Gyro offsets
 		// -----------------------------
-		imu.init_gyro(mavlink_delay);
+		imu.init(IMU::COLD_START, mavlink_delay, &timer_scheduler);
 		#if CLI_ENABLED == ENABLED
 			report_imu();
 		#endif