removing elasticsearch from dependencies

refs: #6343

ElasticSearch will need to be installed separately from 1.1 relase and on.
Rather than maintaining our own custom .deb package, Archivematica will use the
official ElasticSearch deb from packages.elasticsearch.org

Installation instructions have been updated on wiki at
https://www.archivematica.org/wiki/Install-1.1-packages

dev-installer will need to be updated still.