workflows: cascading deletion

* NOTE Adds cascading deletion of BibWorkflowObjects and logs when
  deleting a Workflow.

* Takes advantage of the cascading deletes in test tearDown
  functions - removing the unneeded test_teardown() function.

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>