According to
https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8823, there
are numerous redundant indexes for index_ci_pipelines_on_project_id:

* index_ci_pipelines_on_project_id_and_id_desc
* index_ci_pipelines_on_project_id_and_ref_and_status_and_id
* index_ci_pipelines_on_project_id_and_sha
* index_ci_pipelines_on_project_id_and_source
* index_ci_pipelines_on_project_id_and_status_and_config_source
* index_ci_pipelines_on_project_id_and_status_and_updated_at
* index_ci_pipelines_on_project_idandrefandiddesc

This index is taking 3+ GB, so let's drop it.