Remove appveyor configuration (#23251)

We're no longer using Appveyor.

appveyor.yml

@@ -1,19 +0,0 @@
-environment:
-  matrix:
-    - SHARD: tests
-    - SHARD: tool_tests
-
-install:
-- cmd: cd ..
-- cmd: move flutter "flutter sdk"
-- cmd: cd "flutter sdk"
-- cmd: bin\flutter.bat config --no-analytics
-- cmd: bin\flutter.bat update-packages
-
-cache:
-- '%APPDATA%\Pub\Cache'
-
-build: off
-
-test_script:
-- cmd: bin\cache\dart-sdk\bin\dart.exe -c dev\bots\test.dart