Linux: Revert "Linux: Fix AppImage icon when installing using Joplin_install_and_update.sh (#7346)" (#7559)

This reverts commit cb563f7e60.

Joplin_install_and_update.sh

@@ -216,7 +216,7 @@ then
 	Name=Joplin
 	Comment=Joplin for Desktop
 	Exec=${HOME}/.joplin/Joplin.AppImage ${SANDBOXPARAM} %u
-	Icon=@joplinapp-desktop
+	Icon=joplin
 	StartupWMClass=Joplin
 	Type=Application
 	Categories=Office;

packages/app-desktop/package.json

@@ -95,7 +95,7 @@
       "icon": "../../Assets/LinuxIcons",
       "category": "Office",
       "desktop": {
-        "Icon": "@joplinapp-desktop",
+        "Icon": "joplin",
         "MimeType": "x-scheme-handler/joplin;"
       },
       "target": "AppImage"