Make self monitoring alerting token migration no-op

- Remove it since it will not do anything since the migration that adds
the self-monitoring project itself has been removed.
- The migration does not fail since it checks for the existence of the
self-monitoring project.

db/post_migrate/20190809072552_set_self_monitoring_project_alerting_token.rb

@@ -3,71 +3,17 @@
 class SetSelfMonitoringProjectAlertingToken < ActiveRecord::Migration[5.2]
   DOWNTIME = false
 
-  module Migratable
-    module Alerting
-      class ProjectAlertingSetting < ApplicationRecord
-        self.table_name = 'project_alerting_settings'
-
-        belongs_to :project
-
-        validates :token, presence: true
-
-        attr_encrypted :token,
-          mode: :per_attribute_iv,
-          key: Settings.attr_encrypted_db_key_base_truncated,
-          algorithm: 'aes-256-gcm'
-
-        before_validation :ensure_token
-
-        private
-
-        def ensure_token
-          self.token ||= generate_token
-        end
-
-        def generate_token
-          SecureRandom.hex
-        end
-      end
-    end
-
-    class Project < ApplicationRecord
-      has_one :alerting_setting, inverse_of: :project, class_name: 'Alerting::ProjectAlertingSetting'
-    end
-
-    class ApplicationSetting < ApplicationRecord
-      self.table_name = 'application_settings'
-
-      belongs_to :instance_administration_project, class_name: 'Project'
-
-      def self.current_without_cache
-        last
-      end
-    end
-  end
-
-  def setup_alertmanager_token(project)
-    return unless License.feature_available?(:prometheus_alerts)
-
-    project.create_alerting_setting!
-  end
-
   def up
-    Gitlab.ee do
-      project = Migratable::ApplicationSetting.current_without_cache&.instance_administration_project
+    # no-op
+    # Converted to no-op in https://gitlab.com/gitlab-org/gitlab/merge_requests/17049.
 
-      if project
-        setup_alertmanager_token(project)
-      end
-    end
+    # This migration has been made a no-op because the pre-requisite migration
+    # which creates the self-monitoring project has already been removed in
+    # https://gitlab.com/gitlab-org/gitlab/merge_requests/16864. As
+    # such, this migration would do nothing.
   end
 
   def down
-    Gitlab.ee do
-      Migratable::ApplicationSetting.current_without_cache
-        &.instance_administration_project
-        &.alerting_setting
-        &.destroy!
-    end
+    # no-op
   end
 end

ee/spec/migrations/set_self_monitoring_project_alerting_token_spec.rb

-# frozen_string_literal: true
-
-require 'spec_helper'
-require Rails.root.join('db', 'post_migrate', '20190809072552_set_self_monitoring_project_alerting_token.rb')
-
-describe SetSelfMonitoringProjectAlertingToken, :migration do
-  let(:application_settings) { table(:application_settings) }
-  let(:projects)             { table(:projects) }
-  let(:namespaces)           { table(:namespaces) }
-
-  let(:namespace) do
-    namespaces.create!(
-      path: 'gitlab-instance-administrators',
-      name: 'GitLab Instance Administrators'
-    )
-  end
-
-  let(:project) do
-    projects.create!(
-      namespace_id: namespace.id,
-      name: 'GitLab Instance Administration'
-    )
-  end
-
-  describe 'down' do
-    before do
-      application_settings.create!(instance_administration_project_id: project.id)
-
-      stub_licensed_features(prometheus_alerts: true)
-    end
-
-    it 'destroys token' do
-      migrate!
-
-      token = Alerting::ProjectAlertingSetting.where(project_id: project.id).first!.token
-      expect(token).to be_present
-
-      schema_migrate_down!
-
-      expect(Alerting::ProjectAlertingSetting.count).to eq(0)
-    end
-  end
-
-  describe 'up' do
-    context 'when instance administration project present' do
-      before do
-        application_settings.create!(instance_administration_project_id: project.id)
-
-        stub_licensed_features(prometheus_alerts: true)
-      end
-
-      it 'sets the alerting token' do
-        migrate!
-
-        token = Alerting::ProjectAlertingSetting.where(project_id: project.id).first!.token
-
-        expect(token).to be_present
-      end
-    end
-
-    context 'when instance administration project not present' do
-      it 'does not raise error' do
-        migrate!
-
-        expect(Alerting::ProjectAlertingSetting.count).to eq(0)
-      end
-    end
-  end
-end