Drop index_users_for_auditors for .com

What does this MR do and why?

Drops index_users_for_auditors, which .com has never used. We are keeping it because other customers use features that rely on this index.

See issue #428866 (closed) for details.

Reverting

If we need to recreate this index, it can be done with the following command:

CREATE INDEX CONCURRENTLY index_users_for_auditors ON users USING btree (id) WHERE (auditor IS TRUE);

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #428866 (closed)