[ci] Remove appveyor.yml

Refs: https://github.com/websockets/bufferutil/commit/72a4cd2838299379

.npmignore

@@ -1,4 +1,3 @@
 build/
 test/
-appveyor.yml
 .*

appveyor.yml

@@ -1,18 +0,0 @@
-build: off
-configuration: Release
-environment:
-  matrix:
-    - nodejs_version: '16'
-    - nodejs_version: '14'
-    - nodejs_version: '12'
-platform:
-  - x86
-matrix:
-  fast_finish: true
-install:
-  - ps: Install-Product node $env:nodejs_version $env:platform
-  - npm install
-test_script:
-  - node --version
-  - npm --version
-  - npm test