Removing the id field from the Action trait

This would in theory be useful for a transposition table, but we do not
currently support that. As such I don't want to burden the
implementation with that field until it is deemed necessary.

examples/auto_tic_tac_toe.rs

@@ -84,11 +84,7 @@ struct Move {
     index: usize,
 }
 
-impl Action for Move {
-    fn id(&self) -> usize {
-        self.index
-    }
-}
+impl Action for Move {}
 
 /// Tic-Tac-Toe game state
 #[derive(Clone)]

examples/tic_tac_toe.rs

@@ -119,11 +119,7 @@ struct Move {
     index: usize,
 }
 
-impl Action for Move {
-    fn id(&self) -> usize {
-        self.index
-    }
-}
+impl Action for Move {}
 
 /// Tic-Tac-Toe game state
 #[derive(Clone)]