[board] Remove an unused test helper function

test_hash() was never used. Remove it.

board/src/zobrist.rs

@@ -179,10 +179,6 @@ mod tests {
         ZobristState::new(&mut rng)
     }
 
-    fn test_hash() -> ZobristHash {
-        ZobristHash::new(Arc::new(test_state()))
-    }
-
     #[test]
     fn hash_empty_board_ai_claude() {
         let state = test_state();