src.ctf.fs: remove ctf_fs_ds_file_next

src.ctf.fs: remove ctf_fs_ds_file_next

The function ctf_fs_ds_file_next is not really relevant, as it
doesn't deal with anything ds_file-specific anymore.  It only calls the
ctf_msg_iter_get_next_message and translates the result to a
bt_component_class_message_iterator_next_method_status.

I think this job can (and should) be done by fs.c directly, as it's the
primary user/owner of the ctf_msg_iter.

This patch removes ctf_fs_ds_file_next and updates
ctf_fs_iterator_next_one to call ctf_msg_iter_get_next_message directly.

Change-Id: I96a9e1aa9d3c689bdf19f342464e1632c35058ca
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>