[3.6] Kill the AppVeyor file whitelist (GH-5122)

It's more trouble than it's worth, since AppVeyor only checks the HEAD commit of a PR rather than the full diff against the base branch to decide which files changed.
(cherry picked from commit 7f7de37)

Author: zware

.github/appveyor.yml

@@ -17,20 +17,3 @@ environment:
 image:
 - Visual Studio 2015
 - Visual Studio 2017
-
-# Only trigger AppVeyor if actual code or its configuration changes
-only_commits:
-  files:
-    - .github/appveyor.yml
-    - .gitattributes
-    - Grammar/
-    - Include/
-    - Lib/
-    - Modules/
-    - Objects/
-    - PC/
-    - PCBuild/
-    - Parser/
-    - Programs/
-    - Python/
-    - Tools/