[IA64] fix arch/ia64/mm/contig.c:235: warning: unused variable `nid'

This warning only shows up with CONFIG_VIRTUAL_MEM_MAP=y and
CONFIG_FLATMEM=y.

There is only one caller left for register_active_ranges() from the
contig.c code ... so it doesn't need to pick up the node number, the
node number is always zero.

Signed-off-by: Tony Luck <tony.luck@intel.com>