Deletes gitlab_subscriptions through the app

This is required to avoid an error when re-generating the FK due to
orphaned records. See more https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8871#note_125503270

ee/app/models/ee/namespace.rb

@@ -31,7 +31,7 @@ module EE
       belongs_to :plan
 
       has_one :namespace_statistics
-      has_one :gitlab_subscription
+      has_one :gitlab_subscription, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
 
       scope :with_plan, -> { where.not(plan_id: nil) }
 

ee/spec/models/namespace_spec.rb

@@ -8,7 +8,7 @@ describe Namespace do
   let!(:gold_plan) { create(:gold_plan) }
 
   it { is_expected.to have_one(:namespace_statistics) }
-  it { is_expected.to have_one(:gitlab_subscription) }
+  it { is_expected.to have_one(:gitlab_subscription).dependent(:destroy) }
   it { is_expected.to belong_to(:plan) }
 
   it { is_expected.to delegate_method(:shared_runners_minutes).to(:namespace_statistics) }