SYSDB: Delete linked local user overrides when deleting a user
    
    If a cached user having a linked userOverride is deleted from the LDAP
    server, at some point it will be deleted from cache too but its linked
    userOverride object will not. This causes the command sss_override to
    fail:
    
    [sssd] [append_name] (0x0020): sysdb_search_entry() failed [2]: No such file or directory
    [sssd] [list_overrides] (0x0020): Unable to append name [2]: No such file or directory
    [sssd] [user_export] (0x0020): Unable to get override objects
    [sssd] [override_user_find] (0x0020): Unable to export users
    
    Signed-off-by: Samuel Cabrero <scabrero@suse.de>
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>