[willow] Remove dead `replace_v1_lib_refs_with_v2_in_courses` mgmt command

[kdmccormick]

Remove https://github.com/openedx/openedx-platform/blob/master/cms/djangoapps/contentstore/management/commands/replace_v1_lib_refs_with_v2_in_courses.py and all associated celery tasks and supporting code.

See #38230 (comment) for context