m68k: clean up unused "config ROMVECSIZE"

Kconfig symbol ROMVECSIZE is unused since commit
f84f52a5 ("m68knommu: clean up linker
script"). Let's clean up its Kconfig entry too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>