Revert "Add Windows 8.1 x64 builds to the CI"

This reverts commit ac0c5c20192f2dab979d261a6d5286d57c1d6c15.
Seems to break every MinGW stage.

Change-Id: I10138e9a4865f54b60f8f0d7e9e7befdd0c5d932
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>