Remove no longer required option for association

We already have the ON DELETE CASACADE constraint configured for this FK
at the database level.

ee/app/models/ee/namespace.rb

@@ -27,7 +27,7 @@ module EE
 
       has_one :namespace_statistics
       has_one :namespace_limit, inverse_of: :namespace
-      has_one :gitlab_subscription, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
+      has_one :gitlab_subscription
       has_one :elasticsearch_indexed_namespace
 
       accepts_nested_attributes_for :gitlab_subscription

ee/spec/models/ee/namespace_spec.rb

@@ -14,7 +14,6 @@ RSpec.describe Namespace do
 
   it { is_expected.to have_one(:namespace_statistics) }
   it { is_expected.to have_one(:namespace_limit) }
-  it { is_expected.to have_one(:gitlab_subscription).dependent(:destroy) }
   it { is_expected.to have_one(:elasticsearch_indexed_namespace) }
 
   it { is_expected.to delegate_method(:extra_shared_runners_minutes).to(:namespace_statistics) }