Clean up duo_chat_storage_postgresql feature flags

What does this MR do and why?

Take 2 on Clean up duo_chat_storage_postgresql feature flags (!178671 - merged). This time, we just cleanup duo_chat_storage_postgresql_read and duo_chat_storage_postgresql_write feature flags without dropping the Redis storage.

Later, we'll create a new feature flag in Remove Gitlab::Llm::ChatStorage::Redis storage ... (#501149 - closed) to drop the Redis storage with fixing the false-positive alerts 2025-01-23: All traffic dropped to redis-cluste... (gitlab-com/gl-infra/production#19161 - closed).

Related Remove Gitlab::Llm::ChatStorage::Redis storage ... (#501149 - closed) and [Feature flag] Rollout of `duo_chat_storage_pos... (#506874 - closed)

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• We will introduce duo_chat_drop_redis_storage feature flag in Introduce duo_chat_drop_redis_storage feature f... (!178990 - merged) which associates CR to resolve Remove Gitlab::Llm::ChatStorage::Redis storage ... (#501149 - closed).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.