Investigate if we can drop`index_users_on_email_domain_and_id`

As part of reducing lightweight lock contention, investigate if it's possible to drop the index index_users_on_email_domain_and_id from the projects table

• Find out when and why the index was added
• Decide if the index can be dropped
• Drop the index if possible
• If the index cannot be dropped, document the reason in a comment on this issue and close. /copy_metadata #429092 (closed)