Remove dead code.

Remove dead code.

The renderWithoutShowing was a piece of functionality that we
experimented on long ago and it never quite worked and has
it currently only adds bloat.

It would be sensible to be able to render a window without showing
it on screen, such as for testing purposes, but then it should
be done through proper public API and thouroughly supported
cross platform.

Change-Id: I6bea7335f769c038a8167bad77c2dba171359be9
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>