Doc: Removed "WinRT" from the list of supported platforms.

The port did not make the 5.2 deadline.

Change-Id: I452cb4e16663ab3fd3f8f62c2eb1ef5a56766e97
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>