Some installations somehow gained duplicate indexes on
ci_trigger_requests.trigger_id.

As a result, upgrading to GitLab 12.4.0 may cause this error:

```
ArgumentError: Multiple indexes found on ci_trigger_requests columns
[:trigger_id]. Specify an index name from
ci_trigger_requests_trigger_id_idx, fk_rails_b8ec8b7245,
index_ci_trigger_requests_on_trigger_id
```

This error comes from ActiveRecord's `index_name_for_remove`. This
method raises this exception if the `remove_index` is ambiguous.

Since our end state is to remove all indexes on
`ci_trigger_requests.trigger_id`, find the indexes that have this
signature and pass in an explicit `name` to disambiguate the index to
drop.

Closes https://gitlab.com/gitlab-org/gitlab/issues/34818