Remove appveyor file (#2380)

.appveyor.yml

@@ -1,22 +0,0 @@
-image: Visual Studio 2017
-init:
-- git config --global core.autocrlf true
-branches:
-  only:
-  - dev
-  - /^release\/.*$/
-  - /^(.*\/)?ci-.*$/
-build_script:
-- ps: .\run.ps1 -ci default-build
-on_finish:
-- ps: 7z a testlogs.zip testlogs
-- ps: Push-AppveyorArtifact testlogs.zip
-install:
-- ps: Install-Product node 8
-clone_depth: 1
-environment:
-  global:
-    ASPNETCORE_TEST_LOG_DIR: $(APPVEYOR_BUILD_FOLDER)\testlogs
-test: 'off'
-deploy: 'off'
-os: Visual Studio 2017