Remove AppVeyor integration

[fingolfin]

It is tied to the old repository location. If someone
knows how to update that, of course it could be revived.

(Alternatively, GitHub Actions also allows testing on Windows...)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.64%. Comparing base (6e1e3f4) to head (1b3c8b0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #144   +/-   ##
=======================================
  Coverage   80.64%   80.64%           
=======================================
  Files          11       11           
  Lines        1648     1648           
=======================================
  Hits         1329     1329           
  Misses        319      319           

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