Useless code path in import_from_xml causing errors removed

@dianakhuang @sarina @nasthagiri 

It appears as if a useless code path was causing @carsongee errors while importing course from command line.

@carsongee Can you verify that this fixes your issue?
Since the semantics of the `create_new_course` parameter has now changed, you need to update its description in the docstring above.  Consider even changing its name to something like `create_course_if_not_present`.