src.ctf.fs: remove ctf_fs_ds_file_group_destroy

src.ctf.fs: remove ctf_fs_ds_file_group_destroy

Remove ctf_fs_ds_file_group_destroy and the custom deleter, now that
ctf_fs_ds_file_group can be simply deleted.

Change-Id: I41100be8a99276ef17df4c9aa37a7f4db0275869
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8257
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12295