Terrain following/RTL verify command

[Hwurzburg]

While working on terrain following for AUTOLAND I discovered a few curious things. I think we need a code modification.

1. In AUTO, if the destination altitude is marked as ABOVE_TERRAIN, navigation from the previous waypoint will use TERRAIN following independent of the TERRAIN_FOLLOW bitmask.This is explained in the wiki. Therefore, there is no need for the AUTO enum in TERRAIN_FOLLOW since it has no effect on AUTO mode terrain following and can be removed. There are no mission items that I can find without an alt/frame param (ie non-navigation) that would need this enum to terrain follow, and do_change_alt ignores frame and is only relative alt, nav_return_to-launch switches out of AUTO to RTL, which brings up the next item.
2. I find no code path that actually uses the verify_RTL code. NAV_RETURN_TO_LAUNCH mission item changes out of AUTO mode to pure RTL mode and therefore never calls verify_RTL. I think it can be removed.

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ X] Plane
[ ] Rover
[ ] Submarine