Remove now-unused appveyor config

appveyor.yml

@@ -1,21 +0,0 @@
-# version format
-version: 1.1.3-test_{build}
-
-# pull request template
-pull_requests:
-  do_not_increment_build_number: true
-
-# vm template
-image: Visual Studio 2022
-
-# build step
-build_script:
-  - dotnet build
-
-# success/failure tracking
-on_success:
-  - ps: Invoke-RestMethod https://raw.githubusercontent.com/DiscordHooks/appveyor-discord-webhook/master/send.ps1 -o send.ps1
-  - ps: ./send.ps1 success $env:WEBHOOK_URL
-on_failure:
-  - ps: Invoke-RestMethod https://raw.githubusercontent.com/DiscordHooks/appveyor-discord-webhook/master/send.ps1 -o send.ps1
-  - ps: ./send.ps1 failure $env:WEBHOOK_URL