the trigger by wp distance, trigger with throttle off and trigger a
transistor all don't work, and are not structured correctly, plus are
dangerous.
The existing relay support can (correctly!) handle the resistor case
without hard-wiring a pin. The "turn off throttle to trigger" idea is
badly broken, it would crash a copter if it worked. We can make it
work properly on planes if there is demand. As it was it didn't work
anyway.
The triggger by wp distance method was broken, and had no way to
initiate a trigger anyway
these sends did nothing, as MAVLINK_COMM_3 is not connected to
anything. If it was connected they would be dangerous, as a blocking
serial write could cause the stabilization of a copter not to run. If
the serial port was non-blocking it would corrupt the packet.
If we needed something like this it would have to use the MAVLink
packet queueing logic we use elsewhere