staging/lustre: Drop SLAB_DESTROY_BY_RCU redefine, it's always defined

SLAB_DESTROY_BY_RCU is always defined in kernel slab.h, so
no point in checking for it.

Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>