Remove appveyor from testing

Very old versions of the site were tested with Appveyor,
a service that could test to make sure build steps ran on Windows.
This isn't really necessary now that the site is Dockerized.

Appveyor hasn't run in over 4 years probably when we moved the
organization from pythonsd -> sandiegopython.
I don't think anybody noticed which is a pretty strong indication
it isn't needed.

Author: davidfischer

README.md

@@ -2,8 +2,6 @@
 
 This is the repository for the San Diego Python website at [sandiegopython.org](https://sandiegopython.org).
 
-[![AppVeyor](https://ci.appveyor.com/api/projects/status/184l9lc8y7av2fah?svg=true)](https://ci.appveyor.com/project/davidfischer/pythonsd-django)
-
 
 ## Developing