Draft: Drop index index_namespaces_on_organization_id_for_groups

What does this MR do and why?

Drops the index namespaces.index_namespaces_on_organization_id_for_groups which has been superseded by the newly introduced index namespaces.index_namespaces_on_organization_id_and_id_for_groups in !213392.

References

• #580966
• https://gitlab.com/gitlab-org/database-team/team-tasks/-/issues/560

MR acceptance checklist

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Related to #580966