Remove remenants of mob code

Since minetest has no mobs within the core anymore, I suppose these
settings and code should go. Any mod that uses `minetest.setting_getbool`
will work with no problem since the default return value is `false`.

src/serverobject.h

@@ -97,8 +97,6 @@ class ServerActiveObject : public ActiveObject
 	// If object has moved less than this and data has not changed,
 	// saving to disk may be omitted
 	virtual float getMinimumSavedMovement();
-	
-	virtual bool isPeaceful(){return true;}
 
 	virtual std::string getDescription(){return "SAO";}