Remove foreign key ci_group_variables.group_id as we have loose fK

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77322 we
already added a ["loose foreign key"](
https://docs.gitlab.com/ee/development/database/loose_foreign_keys.html)
to replace this foreign key. Now that this has been running fine in
production we should be OK to remove the foreign key altogether.

This was added in a post-deployment migration as we want to ensure that
they have the correct code running with the loose foreign key in place
before we remove the foreign key to ensure that there isn't any lost
deletes.

Changelog: other

db/post_migrate/20220104060049_remove_foreign_key_ci_group_variables_group_id.rb

+# frozen_string_literal: true
+
+class RemoveForeignKeyCiGroupVariablesGroupId < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  CONSTRAINT_NAME = 'fk_33ae4d58d8'
+
+  def up
+    with_lock_retries do
+      remove_foreign_key_if_exists(:ci_group_variables, :namespaces, name: CONSTRAINT_NAME)
+    end
+  end
+
+  def down
+    add_concurrent_foreign_key :ci_group_variables, :namespaces, column: :group_id, on_delete: :cascade, name: CONSTRAINT_NAME
+  end
+end

db/schema_migrations/20220104060049

+fa87b087b576d320f94028444a082b920870a2554209808849f9f3f42ead83c4
\ No newline at end of file

db/structure.sql

@@ -29179,9 +29179,6 @@ ALTER TABLE ONLY approvals
 ALTER TABLE ONLY namespaces
     ADD CONSTRAINT fk_319256d87a FOREIGN KEY (file_template_project_id) REFERENCES projects(id) ON DELETE SET NULL;
 
-ALTER TABLE ONLY ci_group_variables
-    ADD CONSTRAINT fk_33ae4d58d8 FOREIGN KEY (group_id) REFERENCES namespaces(id) ON DELETE CASCADE;
-
 ALTER TABLE ONLY namespaces
     ADD CONSTRAINT fk_3448c97865 FOREIGN KEY (push_rule_id) REFERENCES push_rules(id) ON DELETE SET NULL;
 

spec/models/ci/group_variable_spec.rb

@@ -42,4 +42,10 @@ RSpec.describe Ci::GroupVariable do
       end
     end
   end
+
+  it_behaves_like 'cleanup by a loose foreign key' do
+    let!(:model) { create(:ci_group_variable) }
+
+    let!(:parent) { model.group }
+  end
 end