The Schema_167 migration was very slow on our production data with about
30K user refs and about 20K groups. This was caused by scanning for all
refs when deleting a ref during migration
When deleting a ref we do not need to check for the conflicting refs
name and can thus avoid the expensive scan for all refs.
Change-Id: I1168bd192fa9c3c8c0791641ef063955d03f35ec
522ffdfaad