lib: remove END values from bt_message_iterator_seek_{beginning,ns_from_origin}_status

The corresponding status enumerations on the component class side:

- bt_component_class_message_iterator_seek_beginning_method_status
- bt_component_class_message_iterator_seek_ns_from_origin_method_status

don't include END, so it's not possible for
bt_self_component_port_input_message_iterator_seek_beginning or
bt_self_component_port_input_message_iterator_seek_ns_from_origin to
return END either.

Change-Id: I57a469d724f9605cd6a169459a39a7286f0e191b
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2015
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>