Revert has_parent? optimization

This reverts optimization made in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/17749

The reason is that at the moment parent_id reference integrity is
not assured and a couple of errors were reported when the referenced
parent was actually missing.

app/models/namespace.rb

@@ -274,7 +274,7 @@ class Namespace < ApplicationRecord
   end
 
   def has_parent?
-    parent_id.present? || parent.present?
+    parent.present?
   end
 
   def root_ancestor