Ignore ProjectAutoDevops domain column

This column is no longer used. The form was removed in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24580
and other usages removed in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28460.

Remove final usages of :domain

app/models/project_auto_devops.rb

 # frozen_string_literal: true
 
 class ProjectAutoDevops < ApplicationRecord
+  include IgnorableColumn
+
+  ignore_column :domain
+
   belongs_to :project
 
   enum deploy_strategy: {
@@ -12,8 +16,6 @@ class ProjectAutoDevops < ApplicationRecord
   scope :enabled, -> { where(enabled: true) }
   scope :disabled, -> { where(enabled: false) }
 
-  validates :domain, allow_blank: true, hostname: { allow_numeric_hostname: true }
-
   after_save :create_gitlab_deploy_token, if: :needs_to_create_deploy_token?
 
   def predefined_variables

spec/controllers/projects/settings/ci_cd_controller_spec.rb

@@ -109,7 +109,7 @@ describe Projects::Settings::CiCdController do
     end
 
     context 'when updating the auto_devops settings' do
-      let(:params) { { auto_devops_attributes: { enabled: '', domain: 'mepmep.md' } } }
+      let(:params) { { auto_devops_attributes: { enabled: '' } } }
 
       context 'following the instance default' do
         let(:params) { { auto_devops_attributes: { enabled: '' } } }

spec/factories/project_auto_devops.rb

@@ -2,7 +2,6 @@ FactoryBot.define do
   factory :project_auto_devops do
     project
     enabled true
-    domain "example.com"
     deploy_strategy :continuous
 
     trait :continuous_deployment do