legal api - remove NAICS endpoint related artifacts (do this last)

Note: This is a placeholder ticket to update necessary BE components when the NAICS service is available as a separate service. It currently resides in the legal api.

**This task should be done last. i.e. after separate NAICS service is up and Create/Edit UI and the legal api is using this new NAICS service**

**TODOs**

- [ ] remove two GET NAICS endpoints (resource and model classes, tests)
- [ ] remove `NAICS_YEAR` and `NAICS_VERSION` from config
- [ ] create alembic script to remove `naics_structures` and `naics_elements` table in `lear` db

First confirm that the separate NAICS service is available and that the Create/Edit UI and legal API use it. Then locate the two GET NAICS resource and model classes and their tests, the configuration entries for NAICS_YEAR and NAICS_VERSION, and the alembic migrations for the lear database. Done means those endpoints, artifacts, configuration values, and the naics_structures and naics_elements tables are removed with tests passing.

Written by the indexing model from the issue text.