[edit export] add required method on export's interface

The method "getFormDefaultData" is applyied on every interface which
will use it:

- ExportInterface
- AggregatorInterface
- DirectExportInterface
- FilterInterface

The method buildForm is moved to those interfaces.

[ci-skip]

src/Bundle/ChillMainBundle/Export/ExportElementInterface.php

@@ -19,10 +19,6 @@ use Symfony\Component\Form\FormBuilderInterface;
  */
 interface ExportElementInterface
 {
-    /**
-     * Add a form to collect data from the user.
-     */
-    public function buildForm(FormBuilderInterface $builder);
 
     /**
      * get a title, which will be used in UI (and translated).