Remove Appveyor CI, replaced with Github actions

.appveyor.yml

-image: Visual Studio 2015
-configuration:
-- Debug
-- DebugDLL
-- DebugDLL_fixed
-- Release
-- ReleaseDLL
-- ReleaseDLL_fixed
-
-platform:
-- Win32
-- x64
-
-environment:
-  api_key:
-    secure: kR3Ac0NjGwFnTmXdFrR8d6VXjdk5F7L4F/BilC4nvaM=
-
-build:
-  project: win32\VS2015\opus.sln
-  parallel: true
-  verbosity: minimal
-
-after_build:
-- cd %APPVEYOR_BUILD_FOLDER%
-- 7z a opus.zip win32\VS2015\%PLATFORM%\%CONFIGURATION%\opus.??? include\*.h
-
-test_script:
-- cd %APPVEYOR_BUILD_FOLDER%\win32\VS2015\%PLATFORM%\%CONFIGURATION%
-- test_opus_api.exe
-- test_opus_decode.exe
-- test_opus_encode.exe
-
-artifacts:
-- path: opus.zip
-
-on_success:
-- ps: if ($env:api_key -and "$env:configuration/$env:platform" -eq "ReleaseDLL_fixed/x64") { Start-AppveyorBuild -ApiKey $env:api_key -ProjectSlug 'opus-tools' }