l2tp: remove l2tp specific refcount debugging

With conversion to refcount_t, such manual debugging code doesn't make
sense anymore.
The tunnel part was already dropped by
54652eb1 ("l2tp: hold tunnel while looking up sessions in l2tp_netlink").

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>