On windows use blink's accessibility implementation

This fixes the windows build after the initial a11y patches landed.
In addition blink a11y works well on this platform so there is no reason
to map it to the Qt one. Since it's based on window handles they mix well.

Change-Id: I4441541864bbdab126cd9412a3640c7284b98193
Reviewed-by: Michael Bruning <michael.bruning@digia.com>