In 1.x, you must define a FormAction.
In Rasa 2.x (currently in alpha), the logic for forms will move from the Rasa SDK to Rasa itself. Then it will be possible to define simple forms without using the Rasa SDK and defining a form action. See here for info about the alpha release (currently 2.0.0.a2) and to try it out. However, it is generally unlikely that you’d have a form with no validation logic or backend work, so even in 2.x, you’re probably going to need to write code for these.