Notify: add pre_arm_gps_check flag

RGB LED will remain flashing blue when vehicle is disarmed and this
check has failed (i.e. false).
Copter: set pre_arm_gps_check flag
Plane: pre_arm_gps_check flag always true
Rover: pre_arm_gps_check flag always true
Tracker: pre_arm_gps_check flag always true
Notify: remove reliance on Led class
Notify: remove Led class

This class didn't add any functionality beyond what the NotifyDevice class
already provided so in keeping with our desire to keep things simple we
can remove this class