Remove references to user_email_lookup_limit

What does this MR do and why?

⚠ This MR will merge into global-search-v15-breaking-changes branch.

Because user_email_lookup_limit is ignored from !80631 (merged), this follows the handbook procedure and removes references to the column: https://docs.gitlab.com/ee/development/avoiding_downtime_in_migrations.html#dropping-columns.

FYI the MR to drop this column is here: !83358 (merged)

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