AP_InertialSensor: remove unused enum bus_speed

The methods actually use the enum from AP_HAL::SPIDeviceDriver, so don't
declare a new one. The I2C implementation is empty; if we actually start
to use it we'd better move the bus abstraction to HAL.

libraries/AP_InertialSensor/AP_InertialSensor_MPU6000.h

@@ -34,9 +34,6 @@ public:
     virtual void init(bool &fifo_mode, uint8_t &max_samples) = 0;
     virtual void read8(uint8_t reg, uint8_t *val) = 0;
     virtual void write8(uint8_t reg, uint8_t val) = 0;
-    enum bus_speed {
-        SPEED_LOW, SPEED_HIGH
-    };
     virtual void set_bus_speed(AP_HAL::SPIDeviceDriver::bus_speed speed) = 0;
     virtual void read_burst(uint8_t* samples,
                             AP_HAL::DigitalSource *_drdy_pin,