shell-ui: remove duplicate actions in gear menu for app menu

These days, GTK+ just shows an app menu on the headerbar.
window-menus: don't special case no-app-menu anymore

We're relying on GTK+ showing the app menu in the header bar these days,
so we don't need this special case anymore.
actions: remove unused GtkActions

We can now remove all the GtkActions that were only used as a proxy
towards the GAction, since we don't expose them anywhere in the UI.

NAUTILUS_ACTION_EDIT_BOOKMARKS is still used for media keys, so keep it
for now.

https://bugzilla.gnome.org/show_bug.cgi?id=735488