check_remove_id/remove_id accepts W_INPUT nodes but doesn't remove the corresponding W_OUTPUT nodes

[tuomas56]

Calling check_remove_id on a W_INPUT node returns true, and remove_id will remove it, leaving the W_OUTPUT node on its own connecting to the other neighbour of the W_INPUT node. I think the expected behaviour here is either to refuse W_INPUT/W_OUTPUT nodes alltogether, or to remove both the W_INPUT node and its partner W_OUTPUT.