neigh: Kill bogus SMP protected debugging message.

Whatever situations make this state legitimate when SMP
also would be legitimate when !SMP and f.e. preemption is
enabled.

This is dubious enough that we should just delete it entirely.  If we
want to add debugging for neigh timer races, better more thorough
mechanisms are needed.

Signed-off-by: David S. Miller <davem@davemloft.net>