src.ctf.fs: remove ctf_fs_ds_file_group_create, add constructor

src.ctf.fs: remove ctf_fs_ds_file_group_create, add constructor

Replace ctf_fs_ds_file_group_create with an equivalent constructor.

Change-Id: I06d69a05ba685552dc2d6b78800d79f20569a340
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8435
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12360
Tested-by: jenkins <jenkins@lttng.org>