Remove TTL support for Elasticsearch in documentation

[davidclement90]

Since 0.2.0 (#79) , ttl is no longer support by Elasticsearch but the doc says that janusgraph-es support it : http://docs.janusgraph.org/latest/elasticsearch.html

[jerryjch]

Ok. This will make JanusGraph's TTL story less complete. We have to think about other ways to support it.