Clean up duplicate indexes on ci_trigger_requests

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

changelogs/unreleased/sh-clean-duplicate-ci-trigger-request-indexes.yml

+---
+title: Clean up duplicate indexes on ci_trigger_requests
+merge_request: 19053
+author:
+type: fixed

db/migrate/20191016072826_replace_ci_trigger_requests_index.rb

@@ -10,7 +10,12 @@ class ReplaceCiTriggerRequestsIndex < ActiveRecord::Migration[5.2]
   def up
     add_concurrent_index :ci_trigger_requests, [:trigger_id, :id], order: { id: :desc }
 
-    remove_concurrent_index :ci_trigger_requests, [:trigger_id]
+    # Some installations have legacy, duplicate indexes on
+    # ci_trigger_requests.trigger_id. Rails won't drop them without an
+    # explicit name: https://gitlab.com/gitlab-org/gitlab/issues/34818
+    old_index_names.each do |name|
+      remove_concurrent_index :ci_trigger_requests, [:trigger_id], name: name
+    end
   end
 
   def down
@@ -18,4 +23,10 @@ class ReplaceCiTriggerRequestsIndex < ActiveRecord::Migration[5.2]
 
     remove_concurrent_index :ci_trigger_requests, [:trigger_id, :id], order: { id: :desc }
   end
+
+  private
+
+  def old_index_names
+    indexes(:ci_trigger_requests).select { |i| i.columns == ['trigger_id'] }.map(&:name)
+  end
 end