powerpc/sched: Remove unused TASK_SIZE_OF

This macro isn't used in Linux sched, now. Delete in
include/linux/sched.h and arch's include/asm.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211228064730.2882351-5-guoren@kernel.org