lib, bt2: graph API: remove "ports connected" listeners

Two ports being connected is always the consequence of the graph user
calling bt_graph_connect_ports() and this function returning
`BT_GRAPH_CONNECT_PORTS_STATUS_OK`. In other words, this event cannot
occur without a direct, concomitant action by the graph user.

Knowing this, the "ports connected" graph listeners are useless.

The "port added" listeners remain useful: a component can add a port at
many moments during the graph configuration phase, therefore having a
"port added" listener can avoid checking if the current graph components
have new ports every time you call bt_graph_add_*_component*() or
bt_graph_connect_ports().

This patch removes everything related to the "ports connected" graph
listeners.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I218c7b7b57c52f2e8589b35e3d89f38dfd961c0a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2636
Tested-by: jenkins <jenkins@lttng.org>