Remove the PDF export menu option

CCBUG:372439
CCMAIL: kimageshop@kde.org

People really should use Scribus (http://scribus.net) to make PDF's,
not Krita, where the the PDF export code depends on QPrinter, which
just isn't powerful enough.

Krita is for creating images; for printing stuff, whether to PDF
or printer, use a dedicated application.