Deprecate QFontDatabase::supportsThreadedFontRendering()

As of 5.0, this always return true.

Change-Id: If225c7219e283da97e870f0d66a87b21c3cacfc0
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>