build: remove AppImage (#294)

BREAKING CHANGE: AppImage is no longer available. This decision has been
made because it has become hard to maintain lately, and I don't have
time nor motivation to fix it.

Author: Deuchnord

.github/workflows/appimage.yml

@@ -27,12 +27,5 @@ prepare-release: messages changelog
 	@echo
 	@echo "Before tagging, don't forget to update version number in CHANGELOG"
 
-appdir:
-	appimage-builder --skip-tests
-	mv *.AppImage dist/
-	mv *.zsync dist/
-
-appimage: appdir
-
 clean:
 	rm -rf build dist appimage-builder-cache kosmorro.egg-info manpage/kosmorro.{1,7}{,.html}