Revert "appveyor ci: look for newer windows sdk version"

This reverts commit 31d632b72e4792c533603db140ca65a45ebdbefe.

according to
https://developer.microsoft.com/en-us/windows/downloads/sdk-archive
10240 is actually 26624 and there was some kind of versioning issue.
f377b1e886Andrew Kelley committed on 1/5/2018, 4:37:21 AMparent7f0b12a
1 file changedLine totals unavailable