Simplify method FilePrinter::psPaperSize

The method FilePrinter::psPaperSize used to contain a long
list of hard-coded paper sizes for the different paper formats.
This patch makes the method get the same information
from QPrinter directly, which is much simpler and more
robust, too.

Note that the method is not actually used anywhere in okular,
though, so this change is purely cosmetic.

REVIEW: 130043