Trimmed the main `elasticsearch.yml` configuration file

This patch significantly trims the main `elasticsearch.yml` configuration file.

The current `elasticsearch.yml` file is a relic of the past, when Elasticsearch
documentation has been more dispersed and incomplete.

It has improved significantly, so the main configuration can be slimmed down,
and contain only the most important settings for development and production use.

Related: 8d0f1a7d123f579fc772e82ef6b9aae08f6d13fd