delete drv_airspeed.h (all IOCTLs), airspeed driver lib

- differential pressure now processed downstream like other sensor data (accel/gyro/mag/etc)
 - rename msg differential_pressure -> sensor_differential_pressure
 - add device id and timestamp_sample to all messages
 - calibration performend directly on raw data and SENS_DPRES_OFF parameter updated
 - remove Airspeed base class from existing differential pressure drivers
 - name differential pressure drivers consistently (no _airspeed suffix)

ROMFS/px4fmu_test/init.d/rc.sensors

@@ -16,11 +16,11 @@ if board_adc start
 then
 fi
 
-if sdp3x_airspeed start -X
+if sdp3x start -X
 then
 fi
 
-if ms5525_airspeed start -X
+if ms5525 start -X
 then
 fi