automation: always use latest Windows AMI
The old AMI isn't available any more.
We seem to run into this problem every few months. Amazon (or
Microsoft) appears to be removing the old AMIs when they are
superseded or something. Let's give up on tracking known images
and switch the image selection logic to use the latest published
image.
Differential Revision: https://phab.mercurial-scm.org/D8465
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors