Doc: remove the note about building qml2puppet

The standalone packages for version 3.0 are built with
Qt 5, so the puppet is always delivered.

Change-Id: I49900ebec6f02314a0bf52dae7cb7c995d2ce402
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>