main: Stop handling old 'panel-main-menu' shortcut

It's a shortcut from the GNOME 2 days that few users will remember.
And if anyone really wants to keep it, they can use Settings to
assign <Alt>F1 to 'toggle-overview'.

https://discourse.gnome.org/t/difference-between-show-the-overview-and-show-the-activities-overview-keyboard-shortcuts/6572

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1871>

js/ui/main.js

@@ -106,10 +106,6 @@ function _sessionUpdated() {
     if (sessionMode.isPrimary)
         _loadDefaultStylesheet();
 
-    wm.setCustomKeybindingHandler('panel-main-menu',
-                                  Shell.ActionMode.NORMAL |
-                                  Shell.ActionMode.OVERVIEW,
-                                  sessionMode.hasOverview ? overview.toggle.bind(overview) : null);
     wm.allowKeybinding('overlay-key', Shell.ActionMode.NORMAL |
                                       Shell.ActionMode.OVERVIEW);