dbus-session: Rename client_vanished() to close()

This means the MetaDbusSession interface takes a more active role
instead of being something that more or less sends signals to the
interface implementor. This will allow better control when using
MetaDbusSession to manage these sessions, instead of their non-abstract
variants.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2713>

src/backends/meta-screen-cast-session.h

@@ -61,8 +61,6 @@ gboolean meta_screen_cast_session_start (MetaScreenCastSession  *session,
 
 gboolean meta_screen_cast_session_is_active (MetaScreenCastSession *session);
 
-void meta_screen_cast_session_close (MetaScreenCastSession *session);
-
 MetaScreenCastStream * meta_screen_cast_session_get_stream (MetaScreenCastSession *session,
                                                             const char            *path);