Postmigrate to drop domain column

Now that the column is ignored, we can safely drop the column

changelogs/unreleased/56959-drop-project_auto_devops_domain.yml

+---
+title: Removes project_auto_devops#domain column
+merge_request: 28574
+author:
+type: other

db/post_migrate/20190522143720_drop_project_auto_devops_domain.rb

+# frozen_string_literal: true
+
+class DropProjectAutoDevopsDomain < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    remove_column :project_auto_devops, :domain, :string
+  end
+end

db/schema.rb

@@ -1632,7 +1632,6 @@ ActiveRecord::Schema.define(version: 20190527194900) do
     t.datetime_with_timezone "created_at", null: false
     t.datetime_with_timezone "updated_at", null: false
     t.boolean "enabled"
-    t.string "domain"
     t.integer "deploy_strategy", default: 0, null: false
     t.index ["project_id"], name: "index_project_auto_devops_on_project_id", unique: true, using: :btree
   end