ARM: 8684/1: NOMMU: Remove unused KTHREAD_SIZE definition

I didn't find any use of this macro in the current kernel tree (with git
grep). KTHREAD_SIZE is no longer used for a very very long time. So
let's remove this definition.

Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>