csky: remove obsolete config CPU_TLB_SIZE

Commit 9d35dc30

 ("csky: Revert mmu ASID mechanism") removes the only
use of CONFIG_CPU_TLB_SIZE. Since then, this config has no effect and can
be deleted.

Remove the obsolete config CPU_TLB_SIZE.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Guo Ren <guoren@kernel.org>