QPrinter - Make PDF support required for QPrinter

Most of QPrinter assumes that QPdf is available as the fall back engine
in case either the plugin fails to load or there are no real printers
configured. Make this assumption explicit in the feature configure and
remove the two places where QT_NO_PDF is actually checked.

Change-Id: Ibb1bdf3bafa5809fbc844c84d7127911a1685506
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>