ardupilotmega.xml: remove FENCE_POINT and RALLY_POINT protocol messages

[peterbarker]

ArduPilot no longer uses the RALLY_POINT and FENCE_POINT messages to transfer rally points around:

// CODE_REMOVAL
// ArduPilot 4.6 sends deprecation warnings for RALLY_POINT/RALLY_FETCH_POINT
// ArduPilot 4.7 stops compiling them in by default
// ArduPilot 4.8 removes the code entirely
#ifndef AP_MAVLINK_RALLY_POINT_PROTOCOL_ENABLED
#define AP_MAVLINK_RALLY_POINT_PROTOCOL_ENABLED HAL_GCS_ENABLED && HAL_RALLY_ENABLED
#endif

We've supported the new transfer mechanism since 4.0 in 2019.

IOW, if we merge this then any GCS which compiles against these definitions will not be able to work with 3.9.x

CubeOrange,-48

(ardupilot size difference)

[khancyr]

why not mark it as deprecated and let another year to remove it ? That would be probably enough to crash all the remaining vehicle on 3.9 serie!