m68k: fix ColdFire mmu init compile warning

Compiling for MMU enabled ColdFire targets gives a warning:

  CC      arch/m68k/mm/mcfmmu.o
arch/m68k/mm/mcfmmu.c: In function ‘paging_init’:
arch/m68k/mm/mcfmmu.c:42:17: warning: unused variable ‘zone’ [-Wunused-variable]
  enum zone_type zone;
                 ^~~~

This was caused by changes in commit fa3354e4

 ("mm: free_area_init:
use maximal zone PFNs rather than zone sizes") leaving around a now
unused variable declaration. Remove the unused variable.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>