AP_HAL: remove state leftover from SPIDeviceDriver

This was previously used to allow to save a state in a SPIDriver so we
could synchronize the initialization of AP_Compass and
AP_InertialSensor.

It was only used by MPU9250 and is not used anymore since the move to
AuxiliaryBus initialization and it's not used anymore since c3dae6f
("AP_InertialSensor: MPU9250: Remove methods not used anymore")

libraries/AP_HAL/SPIDriver.h

@@ -38,15 +38,7 @@ public:
         SPI_SPEED_LOW, SPI_SPEED_HIGH
     };
 
-    enum class State : uint8_t {
-        UNKNOWN,
-        RUNNING,
-        FAILED,
-    };
-
     virtual void set_bus_speed(enum bus_speed speed) {}
-    virtual void set_state(State state) { };
-    virtual State get_state() { return State::UNKNOWN; }
 };
 
 #endif // __AP_HAL_SPI_DRIVER_H__