Fixed youtube stub server and removed unused variable.

Ticket: [TNL-778] (https://openedx.atlassian.net/browse/TNL-778)

If youtube is not available then on importing StubYouTubeService it raises ConnectionError exception.

This needs to be fixed because when youtube is not available and we run lettuce test in which youtube stub server is not used then it raises this exception and we can't run tests.

One of the usages of ```_enroll``` haven't been updated: ```test_course_serializer_with_display_overrides```, fixed it too.