Remove bogus SLAPI code path which special-cased modlist == NULL

Remove bogus SLAPI code path which special-cased modlist == NULL

This had the effect of setting the result code to LDAP_SUCCESS for modlist == NULL
even on a replica when an update-ref might be set. This was causing a plugin (that
implements the commit phase in a post-op plugin) to write to "read-only" replica.