Grammar: remove the dead CommandNode.hint_mode field

HintMode became per-node (Node::Hinted) in the node-attached refactor;
the per-command hint_mode field was never the mechanism and is now
read by nothing. Removed the field and its 20 `None` initialisers.

src/dsl/grammar/mod.rs

@@ -364,8 +364,6 @@ pub struct CommandNode {
     /// carry every variant so the user sees the full family on a
     /// generic-entry-word failure.
     pub usage_ids: &'static [&'static str],
-    #[allow(dead_code)]
-    pub hint_mode: Option<HintMode>,
 }
 
 /// Look up the usage catalog keys for the entry word at the start