Dedented display code. PATTERN.

The pattern ...

    if self._session.display:
        message = ''
        self._io_manager._display(message)

... is no longer necessary.

Just use ...

    self._io_manager._display(message)

... instead.

Why does this work? Because this commit extends IOManager._display() to examine
the Session.display variable.

PATTERN: it is a powerful pattern to pair a session variable (like
Session.display) combined with a context manager (like Interaction) to set and
unset the variable.