CI: removed appveyor

we changed to github actions quite some time ago

appveyor.yml

@@ -1,35 +0,0 @@
-version: 1.0.{build}
-image: Visual Studio 2017 Preview
-clone_depth: 1
-clone_folder: C:\work\
-init:
-- cmd: git config --global core.autocrlf false
-environment:
-  matrix:
-    - TARGET: cygwin64-gcc
-      BUILD_ENVIRONMENT: cygwin64
-      CYG_ROOT: C:\cygwin64
-
-install:
-- cmd: >-
-
-    C:\cygwin64\setup-x86_64.exe -qgnNdO -l C:\cygwin64\var\cache\setup -R c:\cygwin64 -s http://cygwin.mirror.constant.com -P gcc-g++,git,procps,libexpat,python2,python-setuptools,python-future,cygwin32-gcc-g++ )
-
-    dir %CYG_ROOT%
-    dir %CYG_ROOT%\bin
-
-build_script:
-- cmd: >-
-    cd C:\work
-    
-    mkdir c:\work\sitl
-    
-    %CYG_ROOT%\bin\sh.exe /cygdrive/c/work/Tools/scripts/build_appveyor.sh
-    
-    cd c:\work\sitl
-    
-    7z a C:\work\sitl.zip *
-test: off
-artifacts:
-- path: sitl.zip
-  name: sitl