Summary:
DummyANN was created to provide ability to create postgres side part of vector index stuff, while DocDB lack vector index support.
Now DummyANN is not used, and just consumes time in case of refactoring or API changes.

This diff removes DummyANN from DocDB.

**Upgrade/rollback safety:** Marked test only enum entry as deprecated.
Jira: DB-18634

Test Plan: Jenkins

Reviewers: arybochkin

Reviewed By: arybochkin

Subscribers: ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D47396