doc: The lock-ahead translator is not in tree, drop docs

Change-Id: Idda32338af416dc085c48ff023eeacf2397b65fd
Signed-off-by: Lubomir Rintel <lubo.rintel@gooddata.com>
Reviewed-on: http://review.gluster.org/5485
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>