global: remove elastic search 6 support

[lnielsen]

ES6 has reached end of life and we can therefore remove support for ES6.

Search for lt_es7 in the source code to find places with specifics for Elasticsearch 6.

[ghost]

I searched for lt_es7 in the inveniosoftware github-organization and I think, that I found all repositories including lt_es7.
However, I think there is even more, when you are searching for ES_VERSION.

At this point it gets harder and harder to find 'all' so I will stop here and leave the current PRs for review (whenever time and effort is available).