ctf: remove unused `struct ctf_metadata_decoder`

ctf: remove unused `struct ctf_metadata_decoder`

I noticed this struct was unused.  There is another `struct
ctf_metadata_decoder` in src/plugins/ctf/decoder.cpp, that is actually
used.

Change-Id: Ief9a4e37e7414b8fcd729581bb040c1100722327
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8020
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12191
Tested-by: jenkins <jenkins@lttng.org>