[Catalog-sig] Sorting and grouping by multilink properties disabled

["Martin v. Löwis"]

While investigating the load hit on roundup, we found that
Google somewhy issued queries that group by "nosy".

"nosy" being a multilink property, the implementation is a really
lame "download the entire database, sort in Python" approach.

Therefore, I have disabled sorting and grouping by multilink
properties. If that causes problems, please let me know.

Regards,
Martin