LU-6142 llite: remove ll_dir_chain

LU-6142 llite: remove ll_dir_chain

ll_dir_chain is an empty structure that was introduced 13 years ago
and never used.  As such it is clutter.
Remove it an all references to it.  If the need which proposed it
ever does get addresses, having a clean slate in which to create
a suitable implementation is probably preferred.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I1eff80add8851ea11fff979f9b2d25916d6d3eed
Reviewed-on: https://review.whamcloud.com/40290
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>