lib: create_packet_message(): use pool to determine beginning or end msg

The `is_beginning` parameter in create_packet_message() is redundant as
the function can use `pool` and `msg_iter` to determine if it's creating
a packet beginning or end message.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I7f9b4183d004947895fba16b2e8396b8cb5be33a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1363
Tested-by: jenkins
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>