printing: Use original names for remote CUPS printers

printing: Use original names for remote CUPS printers

CUPS uses resource paths in the form of "printers/printer_name"
or "classes/class_name" so it is enough to remove the "printers/"
or "classes/" prefix and use the string behind it as a name.

There was recently introduced a wrong check for the prefix.
This commit fixes it in the way it was originally intended.