AP_Compass: updated backend detect method

Uses reimplemented AK8963 driver. There's also no need trying to detect this
compass twice. Actually, it might even be bad, because the current code can
allocate an AK8963 object twice.

libraries/AP_Compass/Compass.cpp

@@ -351,15 +351,13 @@ Compass::_detect_backends(void)
 
 #if CONFIG_HAL_BOARD == HAL_BOARD_LINUX && CONFIG_HAL_BOARD_SUBTYPE != HAL_BOARD_SUBTYPE_LINUX_NONE
     _add_backend(AP_Compass_HMC5843::detect);
-    _add_backend(AP_Compass_AK8963_MPU9250::detect);
+    _add_backend(AP_Compass_AK8963::detect);
 #elif HAL_COMPASS_DEFAULT == HAL_COMPASS_HIL
     _add_backend(AP_Compass_HIL::detect);
 #elif HAL_COMPASS_DEFAULT == HAL_COMPASS_HMC5843
     _add_backend(AP_Compass_HMC5843::detect);
 #elif HAL_COMPASS_DEFAULT == HAL_COMPASS_PX4 || HAL_COMPASS_DEFAULT == HAL_COMPASS_VRBRAIN
     _add_backend(AP_Compass_PX4::detect);
-#elif HAL_COMPASS_DEFAULT == HAL_COMPASS_AK8963_MPU9250
-    _add_backend(AP_Compass_AK8963_MPU9250::detect);
 #else
     #error Unrecognised HAL_COMPASS_TYPE setting
 #endif