linux/appimage: remove GDK_BACKEND=x11 workaround

This environment variable was necessary back then to work around a bug
in the libgtk3 version we include in the AppImage.

However, since commit 3897a8bccf
("ci/linux: build AppImage on Ubuntu 22.04 (Python 3.10)"), that's not
needed anymore, because Ubuntu 22.04 has a newer version of libgtk3,
which fixes the problem.

I made sure to verify the app runs as expected on Wayland sessions of
two major desktop environments:

- GNOME: 42.9 (Ubuntu 22.04.05) and 48.3 (Arch Linux)
- KDE Plasma: 5.27.5 (Debian 12) and 6.4.3 (Arch Linux)

appimage/AppImageBuilder.yml

@@ -136,9 +136,6 @@ AppDir:
       TCL_LIBRARY: '${APPDIR}/usr/share/tcltk/tcl8.6'
       TK_LIBRARY: '${APPDIR}/usr/lib/tcltk/x86_64-linux-gnu/tk8.6'
       TKPATH: '${APPDIR}/usr/lib/tcltk/x86_64-linux-gnu/tk8.6'
-      # The app seems to have problems running on Wayland at the moment.
-      # See: https://github.com/DevilXD/TwitchDropsMiner/issues/321
-      GDK_BACKEND: x11
 
 AppImage:
   arch: x86_64