SSH2: rm unused constant

it doesn't need to be defined since the strlen($this->last_interactive_response)
check in _login_helper serves the same purpose that NET_SSH_MASK_LOGIN_INTERACTIVE
was intended to serve
Merge pull request #470 from terrafrost/ssh2-rm-unused-constant

SSH2: rm unused constant

* terrafrost/ssh2-rm-unused-constant:
  SSH2: rm unused constant