Remove index issues_on_duplicated_to_id

What does this MR do and why?

Remove index issues_on_duplicated_to_id

Removes issues_on_duplicated_to_id index since we don't use the column for filtering issue lists. The index is not (or barely) used right now.

Changelog: changed

References

• Index Usage: https://dashboards.gitlab.net/goto/Cf1FrtwHR?orgId=1
• Introduced in !16204 (diffs)
• Synchronous removal issue: #557893 (closed)

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