[3.6] bpo-34575: Build with only VS2015 on AppVeyor (GH-9066)

Remove VS2017 image from AppVeyor config.

We're currently having caching issues between the two images, and VS2017 builds are tested by VSTS.  Removing that one here will significantly decrease the time that AppVeyor builds take on this branch.

Author: zware

.github/appveyor.yml

@@ -36,4 +36,3 @@ environment:
   HOST_PYTHON: C:\Python36\python.exe
 image:
   - Visual Studio 2015
-  - Visual Studio 2017