lib: remove self component param. from msg. iterator init. method

Since a3f0c7db ("lib: introduce bt_message_iterator_class"), the
`self_component` parameter of
`bt_message_iterator_class_initialize_method` is useless because you can
access the equivalent with bt_self_message_iterator_borrow_component().

Remove it.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I81e967acfd99b6ef3a2e01ae2ee19008a3c60408
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2761
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>