bt2: make Graph add listener methods return None

bt2: make Graph add listener methods return None

The methods add_ports_connected_listener and add_port_added_listener of
bt2.Graph currently return a _ListenerHandle object.  This is not useful
today, since it's not possible to remove those listeners (unlike trace
class and trace destruction listeners).

Make them not return a handle for now.  It will be possible to make them
return a handle again if/when we add the possibility to remove those
listeners.

Change-Id: I5ea1c9d8793232c2f7077b5c2b82797438066d13
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2424
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>