db: remove unused and rarely used indices

These indices have never been scanned on the production database:

```
 heroku pg:unused_indexes --app travis-production
      table      |               index               | index_size | index_scans
-----------------+-----------------------------------+------------+-------------
 public.commits  | index_commits_on_commit           | 532 MB     |           0
 public.builds   | index_builds_on_state             | 161 MB     |           0
 public.commits  | index_commits_on_branch           | 149 MB     |           0
 public.users    | index_users_on_github_oauth_token | 16 MB      |           0
```

`users.github_oauth_token` is an encrypted column, so no query can be filtered on that column.

The `build(finished_at)`, `jobs(queue,state)`, `jobs(created_at)` and `jobs(repository_id)` indices have very rarely been used:

```
         indexname             | index_size | index_scans
-------------------------------+------------+-------------
 index_builds_on_finished_at   | 131 MB     |          98
 index_jobs_on_queue_and_state | 740 MB     |          19
 index_jobs_on_created_at      | 681 MB     |          38
 index_jobs_on_repository_id   | 580 MB     |          31
```

These indices take up about 3 GB on disk.