staging/lustre: remove alloc_fail_rate sysctl

It was used to control allocation failure rate, but there is
in-kernel way of doing that that's more versatile too.

This is going to remove just the sysctl, the underlying variable
will be removed once all OBD_ALLOC* macros removal patchseries land.

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