Remove milestone foreign-key on boards

db/migrate/20170926202731_add_milestone_foreign_key_to_boards.rb

-class AddMilestoneForeignKeyToBoards < ActiveRecord::Migration
-  include Gitlab::Database::MigrationHelpers
-
-  DOWNTIME = false
-
-  disable_ddl_transaction!
-
-  def up
-    add_concurrent_foreign_key :boards, :milestones, column: :milestone_id, on_delete: :nullify
-  end
-
-  def down
-    remove_foreign_key :boards, column: :milestone_id
-  end
-end

db/schema.rb

@@ -2063,7 +2063,6 @@ ActiveRecord::Schema.define(version: 20170926203418) do
   add_foreign_key "approver_groups", "namespaces", column: "group_id", on_delete: :cascade
   add_foreign_key "board_filter_labels", "boards", name: "fk_53e44f3a07", on_delete: :cascade
   add_foreign_key "board_filter_labels", "labels", name: "fk_91e18fdcee", on_delete: :cascade
-  add_foreign_key "boards", "milestones", name: "fk_b93160e8ee", on_delete: :nullify
   add_foreign_key "boards", "namespaces", column: "group_id", name: "fk_1e9a074a35", on_delete: :cascade
   add_foreign_key "boards", "projects", name: "fk_f15266b5f9", on_delete: :cascade
   add_foreign_key "boards", "users", column: "assignee_id", name: "fk_2a3450e77c", on_delete: :nullify