Remove Outcome#transition method

I don't think this method is serving much purpose. If the `transition` method is
called on an instance of `Outcome`, a `NoMethodError` exception will be raised
which should give just as much (if not more) information than this `InvalidNode`
exception.

Author: floehopper

lib/smart_answer/outcome.rb

@@ -3,9 +3,5 @@ class Outcome < Node
     def outcome?
       true
     end
-
-    def transition(*_args)
-      raise InvalidNode
-    end
   end
 end