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
db: don't remove the jobs(repository_id) index

This index can be useful when debugging, otherwise one would have to
do this via builds.
Merge pull request #350 from travis-ci/hh-indices

db: remove unused and rarely used indices