ctf: make ctf_visitor_generate_ir not use ctf_metadata_decoder_config

ctf: make ctf_visitor_generate_ir not use ctf_metadata_decoder_config

The ctf_metadata_decoder_config structure is going to disappear in the
following patches.  Make ctf_visitor_generate_ir (which we will keep for
now) stop using it.

Change-Id: If4053599d0077554882a9eabe79e4489aa655c72
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8405
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>