navigator: mission check refactor

The mission feasability checker was called with the same arguments
twice which made it hard to understand when a mission is marked valid.

The mission check should run in these two cases:
- When initializing (if home comes up) if there is already a mission saved.
- When the mission gets updated.

src/modules/navigator/mission.h

@@ -211,7 +211,7 @@ private:
 	/**
 	 * Check wether a mission is ready to go
 	 */
-	bool check_mission_valid();
+	void check_mission_valid();
 
 	/**
 	 * Reset offboard mission
@@ -244,7 +244,6 @@ private:
 	} _mission_type;
 
 	bool _inited;
-	bool _home_inited;
 	bool _need_mission_reset;
 
 	MissionFeasibilityChecker _missionFeasibilityChecker; /**< class that checks if a mission is feasible */