mission: The mission check on activation should only be performed for a mission, not RTL.

We need to make sure that when the RTL is triggered, it should not reevaluate it, as when it was valid but evaluated to false on activation, it can't do a RTL.

src/modules/navigator/mission_base.cpp

@@ -207,8 +207,6 @@ MissionBase::on_activation()
 	_mission_has_been_activated = true;
 	_system_disarmed_while_inactive = false;
 
-	check_mission_valid(true);
-
 	update_mission();
 
 	// reset the cache and fill it with the items up to the previous item. The cache contains