Remove all remaining references to AppVeyor (#2451)

It was replaced with .github/workflows/ci-windows-artifacts.yml.
Also remove the broken Travis Build Status link.
diff --git a/README.md b/README.md
index bcd5b70..5d54616 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-# libavif [![AppVeyor Build Status](https://ci.appveyor.com/api/projects/status/github/louquillio/libavif?branch=master&svg=true)](https://ci.appveyor.com/project/louquillio/libavif) [![Travis Build Status](https://travis-ci.com/AOMediaCodec/libavif.svg?branch=master)](https://travis-ci.com/AOMediaCodec/libavif)
+# libavif
 
 This library aims to be a friendly, portable C implementation of the AV1 Image
 File Format, as described here:
@@ -144,12 +144,10 @@
 cmake --build build --parallel
 ```
 
-## Prebuilt Library (Windows)
+## Prebuilt Static Binaries (Windows)
 
-If you're building on Windows with Visual Studio 2022 and want to try out
-libavif without going through the build process, static library builds for both
-Debug and Release are available on
-[AppVeyor](https://ci.appveyor.com/project/louquillio/libavif).
+`avifenc.exe` and `avifdec.exe` can be downloaded from the
+[Releases](https://github.com/AOMediaCodec/libavif/releases) page.
 
 ## Development Notes
 
diff --git a/appveyor.yml b/appveyor.yml
deleted file mode 100644
index a161bd8..0000000
--- a/appveyor.yml
+++ /dev/null
@@ -1,61 +0,0 @@
-image: Visual Studio 2022
-configuration: Release
-skip_non_tags: true
-skip_commits:
-  files:
-    - '.github/*'
-    - '.travis.yml'
-    - '.gitignore'
-    - '*.md'
-
-install:
- # Prepare Visual Studio environment
- - call "C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Auxiliary\Build\vcvars64.bat"
- # Install Nasm
- - appveyor DownloadFile "https://github.com/joedrago/nasm_mirror/raw/master/nasm-2.15.05-win64.zip" -FileName "nasm.zip"
- - 7z x "nasm.zip" > nul
- - move nasm-* NASM
- - set PATH=%PATH%;%CD%\NASM;
- - nasm -v
- # Build all external libraries statically
- - cd ext
- - aom.cmd
- - dav1d.cmd
- - libjpeg.cmd
- - zlibpng.cmd
- - set "CC=" && set "CXX="
- - cd ..
- # Configure with CMake
- - mkdir build
- - cd build
- - cmake --version
- - cmake .. -DAVIF_CODEC_AOM=LOCAL -DAVIF_CODEC_DAV1D=LOCAL  -DBUILD_SHARED_LIBS=OFF -DAVIF_JPEG=JPEG -DAVIF_ZLIBPNG=LOCAL -DAVIF_LIBYUV=LOCAL -DAVIF_BUILD_APPS=ON -DAVIF_ENABLE_WERROR=ON
-
-build:
-  project: build/libavif.sln
-  parallel: true
-  verbosity: minimal
-
-after_build:
- - copy %CONFIGURATION%\avifenc.exe .
- - copy %CONFIGURATION%\avifdec.exe .
- - mkdir lib
- - set ARTIFACT_ZIP=libavif_vs2022_x64_%APPVEYOR_REPO_COMMIT:~0,8%_%CONFIGURATION%.zip
- - copy %CONFIGURATION%\avif.lib lib\avif_x64_%CONFIGURATION%.lib
- - 7z a %ARTIFACT_ZIP% lib
- - cd ..
- - 7z a build\%ARTIFACT_ZIP% include examples
-
-artifacts:
-- path: 'build\libavif_vs2022_x64_*.zip'
-  name: Static Lib and Includes
-- path: 'build\*.exe'
-  name: Basic apps
-
-deploy:
- - provider: GitHub
-   auth_token:
-     secure: 'RsUpI1cVgIyervo85gkQ/qRr9BRx9n6WfqcHrFLB4VNi4BBnfuMt+9dY399oP/EF'
-   prerelease: true
-   on:
-     appveyor_repo_tag: true