Remove redundant index

Now we have cluster_id, state index, we don't the cluster_id index as
well.

db/migrate/20190826090628_remove_redundant_deployments_index.rb

+# frozen_string_literal: true
+
+class RemoveRedundantDeploymentsIndex < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index :deployments, :cluster_id
+  end
+
+  def down
+    add_concurrent_index :deployments, :cluster_id
+  end
+end

db/schema.rb

@@ -1147,7 +1147,6 @@ ActiveRecord::Schema.define(version: 2019_08_28_083843) do
     t.datetime_with_timezone "finished_at"
     t.integer "cluster_id"
     t.index ["cluster_id", "status"], name: "index_deployments_on_cluster_id_and_status"
-    t.index ["cluster_id"], name: "index_deployments_on_cluster_id"
     t.index ["created_at"], name: "index_deployments_on_created_at"
     t.index ["deployable_type", "deployable_id"], name: "index_deployments_on_deployable_type_and_deployable_id"
     t.index ["environment_id", "id"], name: "index_deployments_on_environment_id_and_id"