Drop redundant indexes on the issues table async

What does this MR do and why?

We can save some space by dropping these redundant indexes. These are reported in https://console.postgres.ai/gitlab/gitlab_production/reports/3841/files/622063/md

Also, as part of #536350 (closed), we are creating some indexes so it would be good to drop these so we don't have too many indexes on this table.

References

• #536350 (closed)
• #537598 (closed)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.