panel: Ignore shell-shows-app-menu setting

panel: Ignore shell-shows-app-menu setting

The GtkSettings was originally introduced to inform applications about
the desktop shell's capabilities, but users soon started to use it to
force GTK+ to show the app menu inside the application. We eventually
caved and also handled the setting ourselves to hide the in-shell app
menu to allow users to "move" it.

But now the remote app menu is in the process of being retired[0], and
will be replaced with a simple indicator that cannot be moved, so
stop following the GtkSetting.

[0] https://gitlab.gnome.org/GNOME/Initiatives/wikis/App-Menu-Retirement

https://gitlab.gnome.org/GNOME/gnome-shell/issues/624