remove appveyor

appveyor.yml

@@ -1,28 +0,0 @@
-version: "{build}"
-
-os: Windows Server 2016
-
-image: Visual Studio 2017
-
-clone_folder: c:\gopath\src\github.com\urfave\cli
-
-cache:
-  - node_modules
-
-environment:
-  GOPATH: C:\gopath
-  GOVERSION: 1.11.x
-  GO111MODULE: on
-  GOPROXY: https://proxy.golang.org
-
-install:
-  - set PATH=%GOPATH%\bin;C:\go\bin;%PATH%
-  - go version
-  - go env
-  - go get github.com/urfave/gfmrun/cmd/gfmrun
-  - go mod tidy
-
-build_script:
-  - go run build.go vet
-  - go run build.go test
-  - go run build.go gfmrun docs/v1/manual.md