CI: delete old AppVeyor config (#25373)

I noticed this file as #24892 is updating the NodeJS version in it.

I don't think this is used for anything as the config just runs a handful
of yarn commands and I assume that's all covered by CircleCI.

appveyor.yml

@@ -1,41 +0,0 @@
-image: Visual Studio 2017
-
-# Fix line endings in Windows. (runs before repo cloning)
-init:
-  - git config --global core.autocrlf input
-
-environment:
-  JAVA_HOME: C:\Program Files\Java\jdk1.8.0
-  matrix:
-    - nodejs_version: 10
-
-# Finish on first failed build
-matrix:
-  fast_finish: true
-
-platform:
-  - x64
-
-branches:
-  only:
-    - main
-
-# Disable Visual Studio build and deploy
-build: off
-deploy: off
-
-install:
-  - ps: Install-Product node $env:nodejs_version $env:platform
-  - yarn install --frozen-lockfile
-
-test_script:
-  - node --version
-  - yarn lint
-#  - yarn flow-ci
-  - yarn build
-  - yarn test
-  - yarn prettier
-
-cache:
- - node_modules
- - "%LOCALAPPDATA%/Yarn"