[MTD] remove dead MTD_PMC551_APERTURE_SIZE option

The CONFIG_MTD_PMC551_APERTURE_SIZE option seems to never have existed,
so there's no reason for carrying an #ifdef for it.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>