rcuja: remove "free_node_cb" from destroy

It is now up to the caller to ensure that the Judy array is emptied
before destroying it.

For instance, if nodes are accessible through another data structure
than the Judy array, it may very well be valid to destroy the Judy array
without freeing memory associated to those nodes.

Destroy still takes care of freeing memory used for Judy internal nodes.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>