delete appveyor, no longer necessary (#73)

appveyor.yml

@@ -1,13 +0,0 @@
-version: appveyor.{build}
-image:
-  - Visual Studio 2022
-  - Ubuntu2204
-environment:
-  MINGW_DIR: C:\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0
-install:
-  - cmd: set PATH=%MINGW_DIR%\mingw64\bin;%PATH%
-  - sh: sudo apt update
-  - sh: sudo apt -y install libgtk-3-dev
-build_script:
-  - cmd: mingw32-make.exe TARGET=windows CFLAGS=-O3
-  - sh: make