Remove appveyor.yml (#2466)

The GitHub App integration was removed.

[skip ci]

appveyor.yml

diff --git a/appveyor.yml b/appveyor.yml
deleted file mode 100644
index 180a2c9..0000000
--- a/appveyor.yml
+++ /dev/null
@@ -1,4 +0,0 @@
-# This file exists just to avoid the AppVeyor CI to trigger.
-# See https://github.com/AOMediaCodec/libavif/issues/2448.
-skip_commits:
-  message: /.*/