Remove obsolete dependency on transmogrify.sqlinserter.

`transmogrify.sqlinserter` was used to synchronize users from LDAP into OGDS using transmogrifier.

However, after we rewrote the LDAP import in Python (using SQLAlchemy directly) in #83 we don't need the dependency on `transmogrify.sqlinserter` any more and should hence remove it.