src.ctf.fs: remove ctf_fs_ds_index_destroy

src.ctf.fs: remove ctf_fs_ds_index_destroy

Remove ctf_fs_ds_index_destroy and ctf_fs_ds_index_deleter, since
ctf_fs_ds_index objects can simply be deleted now.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I64eb93eb3f06174523e7f9ceaf94cf103a9e69eb
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8255
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12293