ci: Remove AppVeyor pipeline

We have MSYS2 and MSVC pipelines on GitHub Actions.

.appveyor.yml

@@ -1,67 +0,0 @@
-version: 1.0.{build}
-
-image: Visual Studio 2015
-
-configuration: Release
-
-# Configure both 32-bit and 64-bit builds
-environment:
-  matrix:
-  - platform: x86
-    config: Win32
-    pout: x86
-  - platform: x64
-    config: x64
-    pout: x64
-
-shallow_clone: true
-
-# Download Meson and Ninja, create install directory
-before_build:
-- mkdir build
-- mkdir libepoxy-shared-%pout%
-- cd build
-- curl -LsSO https://github.com/mesonbuild/meson/releases/download/0.47.1/meson-0.47.1.tar.gz
-- 7z x meson-0.47.1.tar.gz
-- move dist\meson-0.47.1.tar .
-- 7z x meson-0.47.1.tar
-- rmdir dist
-- del meson-0.47.1.tar meson-0.47.1.tar.gz
-- curl -LsSO https://github.com/ninja-build/ninja/releases/download/v1.7.2/ninja-win.zip
-- 7z x ninja-win.zip
-- del ninja-win.zip
-- cd ..
-
-# Build and install
-build_script:
-- cd build
-- call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" %PLATFORM%
-- C:\Python36\python.exe meson-0.47.1\meson.py .. . --backend=ninja --prefix=%APPVEYOR_BUILD_FOLDER%\libepoxy-shared-%pout%
-- ninja
-- ninja install
-- cd ..
-
-# Copy license into install directory and create .zip file
-after_build:
-- copy COPYING libepoxy-shared-%pout%
-- dir libepoxy-shared-%pout% /s /b
-- 7z a -tzip libepoxy-shared-%pout%.zip libepoxy-shared-%pout%
-
-artifacts:
-  - path: libepoxy-shared-%pout%.zip
-    name: libepoxy-shared-%pout%
-
-test: off
-
-# Upload .zip file to GitHub release
-deploy:
-  release: $(APPVEYOR_REPO_TAG_NAME)
-  description: "Epoxy $(APPVEYOR_REPO_TAG_NAME)"
-  provider: GitHub
-  auth_token:
-    secure: X7Ro8Y2RWYo/M1AAn93f9X0dEQFvu7gPb6li2eKRtzPYLGj/JKm7MNWRw2cCcjm6
-  artifact: libepoxy-shared-$(pout)
-  draft: false
-  prerelease: false
-  on:
-    appveyor_repo_tag: true # deploy on tag push only