migrations: Remove redundant Postgres indexes

Created by: tsenart

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

On campaign_jobs
campaign_jobs_campaign_plan_id (campaign_plan_id)
is covered by
campaign_jobs_campaign_plan_repo_rev_unique (campaign_plan_id, repo_id, rev)

On discussion_mail_reply_tokens
discussion_mail_reply_tokens_token_idx (token)
is covered by
discussion_mail_reply_tokens_pkey (token)

On discussion_threads
discussion_threads_id_idx (id)
is covered by
discussion_threads_pkey (id)