SITL startup fixes

[bkueng]

• remove non-existing mavlink streams SCALED_IMU2 and SCALED_IMU3. Introduced in Alphabetize the mavlink stream lists in mavlink_main.cpp. #9531 (@mcsauder ). Fixes these warnings:

    WARN  [mavlink] stream SCALED_IMU2 not found
    WARN  [mavlink] stream SCALED_IMU3 not found

• initialize adc only if BOARD_NUMBER_BRICKS > 0. Fixes the SITL startup error:

    ERROR [sensors] no ADC found: /dev/adc0 (9)

@RomanBapst can you test this quickly on Snapdragon, the eagle board config also defines BOARD_NUMBER_BRICKS to 0?

[dagar]

FYI - BOARD_NUMBER_BRICKS defaults to 1 if not set in the board_config.h
https://github.com/PX4/Firmware/blob/master/src/drivers/boards/common/board_common.h#L164-L171

The ADC is also potentially used for differential pressure from the sensors module, although I'm not sure that anyone cares at this point.