Remove replace_v1_lib_refs_with_v2_in_courses dead code

[salman2013]

Summary

• Removes the replace_v1_lib_refs_with_v2_in_courses management command
• Removes its three associated Celery tasks (validate_all_library_source_blocks_ids_for_course, replace_all_library_source_blocks_ids_for_course, undo_all_library_source_blocks_ids_for_course)

This was dead code from an old, abandoned Blockstore-era V2 library migration approach. As noted in #38230, the actual Ulmo migration intentionally leaves source_library_id as the V1 library ID — only the upstream fields point to V2 blocks. Running this command would have incorrectly mutated source_library_id to a V2 ID.

No tests existed for this command/tasks, and no other code references them.

Ticket: #38244

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[farhan]

LGTM!