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>

arch/m68k/mm/mcfmmu.c

@@ -39,7 +39,6 @@ void __init paging_init(void)
 	unsigned long address, size;
 	unsigned long next_pgtable, bootmem_end;
 	unsigned long max_zone_pfn[MAX_NR_ZONES] = { 0 };
-	enum zone_type zone;
 	int i;
 
 	empty_zero_page = (void *) memblock_alloc(PAGE_SIZE, PAGE_SIZE);