Useless/quasi-deprecated fields

[MatMoore]

Should be deprecated ASAP

• document_type (use elasticsearch_document_type)
• id (completely useless)
• updated_at (used inconsistently)
• facet_<FIELD> (use aggregate_<FIELD> instead)

Should be deprecated eventually

• policy_area (in favour of the single taxonomy)
• format (in favour of content_store_document_type and supertypes)
• alert_type, display_type, search_format_types etc (in favour of content_store_document_type)