Remove the need to implement the notification iterator's "get" method

The order of a notification iterator's "get" method is to return the
"current notification", set by the last "next" method call. Since all
source and filter plugins do the same in their "get" method, let's
remove the need to implement it at all.

The bt_notification_iterator_next() and
bt_notification_iterator_get_notification() public functions are still
available for the iterator's user, but the iterator object itself keeps
the current notification returned by the last "next" method call.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>