Remove dead `"BIDDING"` table type and its plumbing

[nick-gorman]

Summary

The "BIDDING" table type is dead code — no table is assigned to it.

defaults.table_types maps every table to one of MMS, FCAS, STATIC, STATICXL, CUSTOM, or DAILY_REGION_SUMMARY. Nothing maps to "BIDDING". The bidding tables (BIDPEROFFER_D, BIDDAYOFFER_D) are type "MMS" and fetch from the MMS monthly archive — not the Reports/Current/Bidmove_Complete/ scrape path the "BIDDING" plumbing was built for.

Because table_type is always resolved via defaults.table_types, every "BIDDING"-keyed entry and code branch is unreachable.

Dead artifacts

• date_generators.bid_table_gen — only referenced via the "BIDDING" entry in processing_info_maps.date_gen.
• downloader.run_bid_tables — only referenced via processing_info_maps.downloader.
• defaults.current_data_page_urls["BIDDING"].
• processing_info_maps.py: the "BIDDING" entries in date_gen, write_filename, and downloader.
• data_fetch_methods._get_read_function: the elif table_type == "BIDDING": branch.
• "BIDDING" in the table-type lists at defaults.py (the dynamic_tables filter) and data_fetch_methods.py (two occurrences).

Note: write_file_names.write_file_names_mms_and_current is not dead — it is still used by DAILY_REGION_SUMMARY. Only the "BIDDING" key referencing it should be removed.

Suggested fix

Remove the artifacts above. Pure dead-code deletion; no behaviour change.

Found while simplifying the test suite's date-generator handling.