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?
I know this code is not new to this PR, but when would this exception ever happen?  The `has_course` method above does not check the `run` either for old Mongo.  So this error case should not occur.  Am I missing something?  (Maybe it did at one point during the evolution of the OpaqueKeys implementation, but `has_course` with `ignore_case=True` also calls the `to_deprecated_son` method which excludes the `run`.