ci: removed deprecated appveyor CI

[fredbi]

• appveyor used to be our sole CI for windows

• this has now been handed over to github actions

• removing this config and the need to manage tokens

• fixes #161

• fixes #167 (duplicate)

[fredbi]

update: needed to remove the appveyor check in the repo settings

[fredbi]

@casualjim if you agree with the principle of deprecating appveyor, I'll remove the webhook from the settings in this repo.