plugins/ctf/common/notif-iter/notif-iter.c: add logging

Remove the logging of the payload returned by the request_bytes() medium
operation of the ctf.lttng.live CC because it is now logged after
calling this user function in the common notif-iter.c.

Make bt_ctf_field_path_string() static in its internal header instead of
BT_HIDDEN because otherwise the plugin cannot access it.

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