Clean up old PostScript printing code

PostScript support was removed back in 2011:
34f712e7



- The QPSPrint* classes are gone
- The QPrinter::OutputFormat enum only has NativeFormat and PdfFormat
  constants
- QPaintEngine::PostScript is unused now

Change-Id: I27d595b92341f41e14bd1e4fd860eac6826d7890
Reviewed-by: Lars Knoll <lars.knoll@digia.com>