models: Remove TODO from 'refresh_tag_counts'

There's actually no harm keeping this around as it makes the code.
Remove the TODO, replacing it with a note on why the function exists.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Reviewed-by: Daniel Axtens <dja@axtens.net>

patchwork/models.py

@@ -296,7 +296,8 @@ class Submission(EmailMixin, models.Model):
     # patchwork metadata
 
     def refresh_tag_counts(self):
-        pass  # TODO(sfinucan) Once this is only called for patches, remove
+        # This is subclassed on 'Patch' to do something useful
+        pass
 
     def is_editable(self, user):
         return False