Remove serialize_opaque_key since CourseLocator and friends contain that functionality now

See OpaqueKey. __unicode__() in opaque-keys/opaque_keys/__init__.py
https://github.com/edx/opaque-keys/blob/master/opaque_keys/__init__.py
Merge pull request #8 from open-craft/opaque-keys-follow-up-gse

Remove serialize_opaque_key