[DwC export]: Remove cache tables when export mapping or extension is deleted

[grantfitzsimmons]

Goal

When an Export Package or its extension mapping is deleted, automatically remove the corresponding cache table(s) from the database.

Note: This complements #7738, which handles orphan cleanup. This issue focuses on the delete hook.

Acceptance Criteria

• Deleting an exportdataset record triggers removal of all associated cache tables (core + extensions).
• Deleting an entry from the extensions join table removes only that extension's cache table.
• The deletion is handled in a post-delete hook or signal to ensure consistency.
• If the cache table does not exist at deletion time, the operation succeeds silently.