percpu: remove PERCPU_ENOUGH_ROOM which is stale definition

As pure cleanup, this patch removes PERCPU_ENOUGH_ROOM which is not
used any more. That is, no code refers to the definition.

Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Jungseok Lee <jungseoklee85@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>