refactor: simplify removeconstignore verify (#768)

leftover review comment:
#757 (comment)

src/hugr/rewrite/consts.rs

@@ -45,11 +45,12 @@ impl Rewrite for RemoveConstIgnore {
  if (!h.contains_node(node)) || (!h.get_optype(node).is_load_constant()) {
  return Err(RemoveError::InvalidNode(node));
  }
-
- if h.out_value_types(node)
- .next()
- .is_some_and(|(p, _)| h.linked_inputs(node, p).next().is_some())
- {
+ let (p, _) = h
+ .out_value_types(node)
+ .exactly_one()
+ .ok()
+ .expect("LoadConstant has only one output.");
+ if h.linked_inputs(node, p).next().is_some() {
  return Err(RemoveError::ValueUsed(node));
  }