AP_InertialSensor: remove out of place ifdef

AP_InertialSensor isn't a good place to comment about board issues.

libraries/AP_InertialSensor/AP_InertialSensor.cpp

@@ -424,13 +424,6 @@ AP_InertialSensor::_detect_backends(void)
     #error Unrecognised HAL_INS_TYPE setting
 #endif
 
-#if 0 // disabled due to broken hardware on some PXF capes
-#if CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_PXF
-    // the PXF also has a MPU6000
-    _add_backend(AP_InertialSensor_MPU6000::detect);
-#endif
-#endif
-
     if (_backend_count == 0 ||
         _gyro_count == 0 ||
         _accel_count == 0) {