ci: remove AppVeyor workflow

[MikeMcC399]

Issue

• The CI workflow defined by .appveyor.yml is outdated and failing.
• The last failing message was:

  The build phase is set to "MSBuild" mode (default), but no Visual Studio project or solution files were found in the root directory. If you are not building Visual Studio project switch build mode to "Script" and provide your custom build command.

• The link https://ci.appveyor.com/project/indexzero/ps-tree/history to AppVeyor results is not included in the README, nor is there a badge for AppVeyor in the README.
• CI checks on Windows are now carried out with the GitHub Actions workflow ci-test.yml

Change

Remove the .appveyor.yml workflow.

Follow-on action

• Check if there is a need to delete any remaining information or account data from https://ci.appveyor.com/project/indexzero/ps-tree

[nelsonic]

@MikeMcC399 Thanks for removing this stale file. ✂️
Appveyor no longer supports these legacy versions of Node.js. 🙅