remove appveyor CI

appveyor.yml

-image: Visual Studio 2019
-
-environment:
-  ICUWIN: '%APPVEYOR_BUILD_FOLDER%\icu'
-
-install:
-  # Set paths to dependencies (based on architecture)
-  - cmd: set PYTHON_ROOT=C:\python37-x64
-  # Add necessary paths to PATH variable
-  - cmd: set PATH=%cd%;%PYTHON_ROOT%;%PYTHON_ROOT%\Scripts;%PATH%
-  # Set up the build environment
-  - cmd: set BOOST_ROOT=C:\Libraries\boost_1_73_0
-  - cmd: '"C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat" x86_amd64'
-  # Install meson and ninja
-  - cmd: pip install meson
-  - cmd: chdir %APPVEYOR_BUILD_FOLDER%
-  - cmd: curl -LO https://github.com/ninja-build/ninja/releases/download/v1.10.0/ninja-win.zip
-  - cmd: unzip ninja-win.zip
-
-build_script:
-  - cmd: meson -Ddefault_library=static --backend=ninja --buildtype release -Dicu_path=%ICUWIN% builddir
-  - cmd: ninja -C builddir
-