m68k: coldfire: remove unused variable in MMU code

When building with W=1:

  CC      arch/m68k/mm/mcfmmu.o
arch/m68k/mm/mcfmmu.c: In function ‘paging_init’:
arch/m68k/mm/mcfmmu.c:41:30: warning: variable ‘bootmem_end’ set but not used [-Wunused-but-set-variable]
  unsigned long next_pgtable, bootmem_end;
                              ^~~~~~~~~~~

Remove variable bootmem_end and its unused setting.

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