staging/lustre: Remove unused ptlrpcd_add_rqset()

No point in retaining it if it's unused.

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