Remove the primary_sector_single legacy field identifier

[maebeale]

Follow-up to #2275.

#2275 makes primary_sector the canonical single-select primary sector field identifier, while temporarily keeping primary_sector_single accepted as a legacy alias so existing forms keep resolving.

Once #2275 ships and prod form data is migrated from primary_sector_single → primary_sector, the alias can be dropped entirely.

Sequencing

1. Merge Consolidate sector field identifiers to primary_sector + additional_sectors #2275 (code accepts both primary_sector and primary_sector_single).
2. Migrate prod: rename stored form_fields.field_identifier primary_sector_single → primary_sector.
3. Merge the removal PR (drops primary_sector_single from the code).

Removal scope

• FormField::PRIMARY_SECTOR_FIELD_IDENTIFIERS → %w[primary_sector]
• Drop the primary_sector_single "(legacy)" row from SmartFormFields
• Delete the legacy_form_identifiers dev seed + rake task (no legacy identifiers remain once this is gone)

A draft PR for the removal is prepared and must not merge until step 2 is done.