Remove legacy note attachment code

What does this MR do and why?

These have been deprecated and migrated years ago so these code can now be removed.

For context, a Note used to have an associated Upload in case a user attaches something to a note. Today, we associate uploads with Project / Group and then generate the link for the Markdown within the note or description.

The migration was done many years ago but the code was never removed.

References

Related to #20830 (closed)

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