ci: remove appveyor

(cherry picked from commit 3ce31df3)

appveyor.yml

-version: '{build}'
-branches:
-  only:
-  - master
-  - appveyor
-  - /^maint.*/
-environment:
-  GITTEST_INVASIVE_FS_STRUCTURE: 1
-  GITTEST_INVASIVE_FS_SIZE: 1
-
-  matrix:
-  - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2013
-    CMAKE_OPTIONS: -G"Visual Studio 10 2010"
-    ARCH: x86
-  - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2013
-    CMAKE_OPTIONS: -G"Visual Studio 10 2010 Win64"
-    ARCH: amd64
-  - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-    CMAKE_OPTIONS: -G"Visual Studio 14 2015"
-    ARCH: x86
-  - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-    CMAKE_OPTIONS: -G"Visual Studio 14 2015 Win64"
-    ARCH: amd64
-  - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-    CMAKE_OPTIONS: -G"MinGW Makefiles"
-    ARCH: x86
-  - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-    CMAKE_OPTIONS: -G"MinGW Makefiles"
-    ARCH: amd64
-
-install:
-- set PATH=%TEMP%\mingw64\bin;%TEMP%\mingw32\bin;C:\ProgramData\Oracle\Java\javapath;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\CMake\bin
-- ps: |
-    mkdir build
-    cd build
-    if ($env:CMAKE_OPTIONS -eq '-G"MinGW Makefiles"') {
-      ../ci/setup-mingw.ps1
-    }
-
-build_script:
-- cmd: powershell ../ci/build.ps1
-
-test_script:
-- cmd: powershell ../ci/test.ps1