remove a workaround to install Node.js v12 on AppVeyor

appveyor/ci#2921 has been fixed.

Author: shinnn

appveyor.yml

@@ -3,8 +3,7 @@ platform: x64
 shallow_clone: true
 skip_tags: true
 install:
-  # https://github.com/appveyor/ci/issues/2921
-  - ps: Update-NodeJsInstallation (Get-NodeJsLatestBuild 12) x64
+  - ps: Install-Product node '' x64
   - npm ci
 build: off
 test_script: node test.js