Run the ND6 expiry timer without kernel lock

Run the ND6 expiry timer without kernel lock

Added in 2017 to
	Reduce contention on the NET_LOCK() by moving the nd6 address expiration
	task to the `softnettq`.

This should no longer be needed thanks to sys/net/if.c r1.652 in 2022:
	Activate parallel IP forwarding.  Start 4 softnet tasks.  Limit the
	usage to the number of CPUs.

Nothing in nd6_expire() or nd6_expire_timer_update() requires protection by
the kernel lock.

The interface list and per-interface address lists remain protected by the
net lock.

Tests by Hrvoje
OK mpi