Remove the old AppImage structure

These files did not serve a real purpose since `icon.png` can be used
as the AppImage icon, and a .desktop file can be found at
`misc/dist/linux/godot.desktop` with the same contents.

misc/dist/appimage/AppRun

@@ -1,3 +0,0 @@
-#!/bin/sh
-HERE="$(dirname "$(readlink -f "${0}")")"
-"${HERE}"/godot $@

misc/dist/appimage/godot.desktop

@@ -1,9 +0,0 @@
-[Desktop Entry]
-Name=Godot Engine
-GenericName=Libre game engine
-Comment=Multi-platform 2D and 3D game engine with a feature rich editor
-Exec=godot -pm
-Icon=godot
-Terminal=false
-Type=Application
-Categories=Development;IDE;