numa: remove stale node_has_online_mem() define

This isn't used anywhere, so delete it.

Looks like the last usage (in x86-specific code) was removed by Tejun
in 2011 in commit bd6709a9

 ("x86, NUMA: Make 32bit use common NUMA
init path").

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>