classic: Account for removal of app menu

We no longer include the app menu in the regular GNOME session,
we shouldn't bring it back in Classic (not least because there
never was a similar menu in GNOME 2).

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/255>

data/classic.json.in

@@ -4,7 +4,7 @@
     "hasOverview": false,
     "showWelcomeDialog": false,
     "enabledExtensions": [@CLASSIC_EXTENSIONS@],
-    "panel": { "left": ["appMenu"],
+    "panel": { "left": [],
                "center": [],
                "right": ["a11y", "keyboard", "dateMenu", "quickSettings"]
              }