txn: kill txn_last_stmt helper

It's not used anywhere anymore.

src/box/txn.h

@@ -416,13 +416,6 @@ txn_current_stmt(struct txn *txn)
 	return stailq_entry(stmt, struct txn_stmt, next);
 }
 
-/** The last statement of the transaction. */
-static inline struct txn_stmt *
-txn_last_stmt(struct txn *txn)
-{
-	return stailq_last_entry(&txn->stmts, struct txn_stmt, next);
-}
-
 /**
  * FFI bindings: do not throw exceptions, do not accept extra
  * arguments