Remove Appimage support

We have decided to drop support for Appimage because it was causing
issues to too many users.

docs/CHANGELOG.md

@@ -49,6 +49,10 @@ and this project adheres to
 
   - Default theme is now Foliage.qpl
 
+### Removed
+
+- Appimage: remove Appimage installation method
+
 ### Fixed
 
   - Compatibility with Python 3.10

docs/INSTALL.md

@@ -5,7 +5,6 @@ but compiling on Windows and macOS should be possible with the right tools.
 
 - [Packages](#packages)
   - [Linux](#linux)
-    - [AppImage](#appimage)
     - [Flatpak](#flatpak)
     - [Alpine Linux / postmarketOS](#alpine-linux--postmarketOS)
     - [Arch Linux](#arch-linux)
@@ -37,22 +36,9 @@ but compiling on Windows and macOS should be possible with the right tools.
 
 For developement, or if none of the package options are satisfying, 
 see [manual installation](#manual-installation).  
-Packages other than the AppImage and Flatpak are not maintained by the Mirage 
+Packages other than the Flatpak are not maintained by the Mirage 
 authors, and thus might be outdated.
 
-#### AppImage (Leftover instructions from Mirage; not supported for Moment)
-
-For **x86 64bit glibc-based systems**, Mirage is available as an AppImage
-on the [release page](https://github.com/mirukana/mirage/releases).
-
-AppImages are single executable files that contain the app and all 
-its dependencies.  
-Mirage images are built in Ubuntu 16.04, and should therefore run on any distro
-released in April 2016 or later.
-
-[How to start AppImages](https://docs.appimage.org/introduction/quickstart.html#how-to-run-an-appimage)
-(TL;DR: `chmod +x Mirage-*.AppImage && ./Mirage-*.AppImage`)
-
 #### Flatpak (Leftover instructions from Mirage; not supported for Moment)
 
 Mirage is also available as a Flatpak.