Remove appveyor

[IAlibay]

Fixes #3065

Changes made in this Pull Request:

• Quite simply; removes appveyor from MDA

I'll raise another issue about adding a conda run to the azure matrix (or we can add Windows to GH actions).

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[IAlibay]

How odd, the webhook is still active even if you don't have the yaml file around 🙃

I can delete it through, but I guess I should wait to hear any dissenting opinions on the issue/PR first?

[tylerjereddy]

Makes sense, Oli was in favor of this based on the matching issue and sounds like there is a plan in place to have a conda-based Windows run somewhere to make up for the dependency testing.

Probably safe to get rid of the Appveyor web hook too

[IAlibay]

I've gone ahead deleted the webhook now, we just need to remove the yaml file from master too.