SDAP: Remove user from cache for missing user in LDAP
    
    Function sysdb_get_real_name overrode reurned code LDAP
    and thus user was not removed from cache after removing it from LDAP.
    This patch also do not try to set initgroups flag if user
    does not exist. It reduce some error message.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2681
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>