remove appveyor.yml -> replaced by github CI action

[chcg]

@donho Likely you also need to remove this plugin from appveyor to avoid the failing appveyor build.

[donho]

@chcg Is it normal this failure? https://ci.appveyor.com/project/donho/mimetools/builds/47160914

[chcg]

@donho You need to got to https://ci.appveyor.com/project/donho/mimetools/settings and delete the project from appveyor togother with the history. If you just want to stop building likely disabling:

• Do not build on "Push" events
• Do not build on "Pull request" events is sufficient