Ticket 49257 - only register modify callbacks
    
    Bug Description:  Regression.  In the previous fix we called
                      ldbm_instance_config_load_dse_info() to register all
                      the dse preop callbacks.  Previously this was only done
                      when creating an instance.  It was not designed to be
                      used outside of that context, and it caused error 53's
                      when trying to add a backend after instance creation.
    
    Fix Description:  Just register the "modify" DSE preop callbacks.
    
    https://pagure.io/389-ds-base/issue/49257
    
    Reviewed by: ?
    
    (cherry picked from commit 75a32a8829297a5cab303590d049f581740cf87e)