sink.ctf.fs: remove unused `fs_sink_ctf_field_class::index_in_parent` field

sink.ctf.fs: remove unused `fs_sink_ctf_field_class::index_in_parent` field

This field is not used anywhere and looks like a leftover from a
previous iteration of the sink. Let's remove it.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I64592c1561bd66e083e965903d8a9dc050a5756d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7387
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12189
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Simon Marchi <simon.marchi@efficios.com>