m68knommu: remove no longer used rom_length from Palm/Pilot start up code

Compiling for the m68knommu/68328 Palm/Pilot target you get:

  LD      vmlinux
arch/m68k/platform/68328/head.o: In function `L3':
(.text+0x170): undefined reference to `rom_length'

"rom_length" is not used any longer by any of the m68knommu code.
So remove it from here too.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>