campaigns: unused index campaign_jobs_campaign_plan_id

Created by: keegancsmith

According to PgHero we have an unused index.

These indexes exist, but aren’t needed. Remove them for faster writes

On patches campaign_jobs_campaign_plan_id (patch_set_id) is covered by campaign_jobs_campaign_plan_repo_rev_unique (patch_set_id, repo_id, rev)

cc @sourcegraph/campaigns-core