QImageReader: simplify _qt_BuiltInFormats

By construction of the _qt_BuiltInFormatType enum, the .type in
_qt_BuiltInFormats exactly equals its position. So there's no
need to store it explicitly.

Effects on Linux AMD64 GCC 4.9-trunk release stripped:
  text: -16B
  data: -64B
  relocs: +-0 (of course)

Change-Id: Ib68c32de42d264f27bbd8a863538dd78b1ba4507
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>