ArduPlane failsafes: remove rc_override_active

- rc_override_active is never set anywhere in the ArduPlane code; it's only used for Copter and Rover. Removing it significantly simplifies the failsafe code.
- modified code has been tested in SITL. Heartbeat and RC failures in AUTO, CRUISE, and RTL modes (covering the three cases in the failsafe check functions) have been simulated with FS_LONG_ACTN = 0, 1, and 2, FS_SHORT_ACTN = 0, 1, and 2, and FS_GCS_ENABL = 0, 1, and 2. In all cases tested, the results are identical to those with the original code.