Rename start and end fields (4.1/4)
Rename start and end fields (4.1/4)

This is the 4.1th stage of renaming the start and end fields of
CourseRun to start_date and end_date.

This release ONLY removes the django model fields corresponding to the
old columns.  Note that this does not include the migration to remove
the columns.

DE-1708

edx/credentials,edx/credentials,edx/credentials,edx/credentials