Investigate if we can drop `index_users_for_auditors`

Preliminary investigation:

1. As stated in the epic, this has been scanned zero times ever.
2. It was added in !126064 (merged). There are no query plans, however.
3. It's only a few kilobytes
4. It's only useful for SELECT * FROM users WHERE auditor IS TRUE or some such query.

I have inquired on the original MR as to the nature of this index.

Update: According to this thread this index is being used extensively by at least one self-hosted installation.