Removing unused code in Field, Step models.

* Improving unittests for step, "fixing" errors in get and delete
* Removing useless functions in Field, as they are not really useful for
  handlers.
Field deletion shall be recursive, and include steps whenver is the case.

Fix model to properly delete first childs and then itself.
Fix handler to reflect those changes, and act also on the step associated
whenever is the case.
Warning: still have to make unittests on this.