appveyor.yml: Disable everything except Windows builds

Nobody should be using these anymore, but just in case, it'll make it
complete a bit quicker.

appveyor.yml

@@ -6,27 +6,11 @@ skip_tags: true
 
 image:
 - Visual Studio 2019
-- Ubuntu1804
-- macOS
 
 install:
 - cmd: >-
     git submodule update --init --depth 1
     
-- sh: >-
-    if [ "$APPVEYOR_BUILD_WORKER_IMAGE" == "Ubuntu1804" ]; then
-
-      sudo apt-get update
-
-      sudo apt-get install -y cmake ninja-build ccache libsdl2-dev libgtk2.0-dev qtbase5-dev qtbase5-dev-tools qtbase5-private-dev qt5-default
-
-    elif [ "$APPVEYOR_BUILD_WORKER_IMAGE" == "macOS" ]; then
-
-      brew install qt5 sdl2
-
-    fi
-
-
 build_script:
 - cmd: >-
       call "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat" x64
@@ -44,46 +28,4 @@ build_script:
       appveyor PushArtifact duckstation-win64-release.7z
 
 
-- sh: >-
-    if [ "$APPVEYOR_BUILD_WORKER_IMAGE" == "Ubuntu1804" ]; then
-
-      mkdir -p build-release
-
-      cd build-release
-
-      cmake -DCMAKE_BUILD_TYPE=Release -DBUILD_SDL_FRONTEND=ON -DBUILD_QT_FRONTEND=ON -DUSE_SDL2=ON -G Ninja ..
-
-      ninja
-
-      ../appimage/generate-appimages.sh $(pwd)
-
-      if [ $? -eq 0 ]; then
-
-        mv duckstation-qt-x64.AppImage duckstation-qt-x64-release.AppImage
-
-        mv duckstation-sdl-x64.AppImage duckstation-sdl-x64-release.AppImage
-
-        7za a -r duckstation-linux-x64-release.7z duckstation-*.AppImage
-
-        appveyor PushArtifact duckstation-linux-x64-release.7z
-
-      else
-
-        echo "Failed to create AppImages, no AppImage artifact will be pushed"
-
-      fi
-
-    elif [ "$APPVEYOR_BUILD_WORKER_IMAGE" == "macOS" ]; then
-
-      mkdir build-release
-
-      cd build-release
-      
-      cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DBUILD_SDL_FRONTEND=YES -DBUILD_QT_FRONTEND=YES -DQt5_DIR=/usr/local/opt/qt/lib/cmake/Qt5 ..
-
-      make
-      
-    fi
-
-
 test: off