Clean up old kit table and items.kit_id column (#5679)

[aniketpatidar]

Resolves #5679

Description

• Deletes the old, unused kits table and removes the kit_id column from items.
• Removes old foreign key references so the table can be safely deleted without errors.

Type of change

• Clean up

How Has This Been Tested?

• Run bundle exec rails db:migrate and verify clean schema dump.
• Run relevant test suite: bundle exec rspec spec/models/kit_spec.rb spec/system/kit_system_spec.rb spec/requests/kit_requests_spec.rb spec/services/kit_create_service_spec.rb spec/models/item_spec.rb
• Verify creating, updating, and allocating kits still works properly.

[aniketpatidar]

Please review #5723, which cleans up the old kit table and column. Thanks!

@dorner