CI/AppVeyor: revert whitelist

revert 6b028ec neovim#10746

This seems to skip tags also, which breaks our release automation.
From the AppVeyor logs:

    2019-09-15 17:25:00.232 Warning Commit "1f76c4af" of branch "master" skipped as commit message contains either "[skip ci]" or "[ci skip]" or "[skip appveyor]"
    2019-09-15 17:20:24.152 Warning Commit "baa5263b" skipped as branch "stable" is not in white-list
    2019-09-15 17:20:03.163 Warning Commit "6cb5ffc0" skipped as branch "release-0.4" is not in white-list
    2019-09-15 17:20:02.977 Warning Commit "baa5263b" skipped as branch "v0.4.1" is not in white-list
    2019-09-15 16:35:38.810 Warning Commit "31956372" of branch "master" skipped as commit message contains either "[skip ci]" or "[ci skip]" or "[skip appveyor]"
    2019-09-15 16:28:35.237 Warning Commit "e2cc5fe0" skipped as branch "stable" is not in white-list
    2019-09-15 16:28:17.814 Information Build version 31489 created
    2019-09-15 16:28:16.364 Warning Commit "e2cc5fe0" skipped as branch "v0.4.0" is not in white-list

appveyor.yml

@@ -40,6 +40,3 @@ cache:
 artifacts:
 - path: build/Neovim.zip
 - path: build/bin/nvim.exe
-branches:
-  only:
-    - master