s390: remove orphaned extern variables declarations

arch/s390/kernel/entry.h: suspend_zero_pages - only declaration left
after commit 39421627 ("s390: remove broken hibernate / power
management support")

arch/s390/include/asm/setup.h: vmhalt_cmd - only declaration left after
commit 99ca4e58 ("[S390] kernel: Shutdown Actions Interface")

arch/s390/include/asm/setup.h: vmpoff_cmd - only declaration left after
commit 99ca4e58

 ("[S390] kernel: Shutdown Actions Interface")

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>