staging/lustre/llog: remove unused server code

Most of the code in llog.c and llog_cat.c is completely unused
on the client and can be removed, as a preparation for removing
the dt_object code.

Two tricky parts are:

 - In llog_cat_close(), we rely on the fact that llh_flags never
   contains LLOG_F_ZAP_WHEN_EMPTY, because nobody ever sets that
   flag.

 - In llog_read_header(), we check the return value of the lpi_cb
   callback, and again we know that it cannot be LLOG_PROC_BREAK
   or LLOG_DEL_RECORD and can remove the respective code path.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>