Remove Windows 8 from CI

Change-Id: Iba49f297cade4594689295a19b2ba27cbf084327
Reviewed-by: Simo Fält <simo.falt@digia.com>
Add former Win 8 configurations to be built in Win 8.1

Following projects are built for Windows 8.1:
QtAndroidExtras_dev / stable / release
QtWebKit_dev / stable / release
QtWinExtras_dev / stable / release

Change-Id: Iaddb6d86100971fa93aa18307036b6248eea561e
Reviewed-by: Simo Fält <simo.falt@digia.com>