EnterInterpOnlyModeClosure::completed() always returns true

JDK-8238761 introduced this funky code:

class EnterInterpOnlyModeClosure : public HandshakeClosure {
 private:
  bool _completed;
 public:
  EnterInterpOnlyModeClosure() : HandshakeClosure("EnterInterpOnlyMode"), _completed(false) { }
  void do_thread(Thread* th) {
     ...
     _completed = true;
  }
  bool completed() {
    return _completed = true;
  }
};

It seems the flag is there to communicate that target thread indeed executed the handshake. But `completed()` sets the bool unconditionally and always returns true. And it is used in one and only place here:

     guarantee(hs.completed(), "Handshake failed: Target thread is not alive?");

...which means that guarantee always passes.