Consider removing index_mr_metrics_on_target_project_id_merged_at_time_to_merge index

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index_mr_metrics_on_target_project_id_merged_at_time_to_merge was introduced on !46040 (merged), but execution times are quite similar with and without the index, see !46040 (comment 458368539) for more details.

To help with our index bloat https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9563 we should consider removing this index