staging/lustre: Remove high-priority request callbacks

This function is only used on the server where real high-priority
requests actually exist.
This deletes ptlrpc_hpreq_handler() and ptlrpc_request_change_export()

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