Remove Ruby elasticsearch repository indexer
Currently, GitLab's Elasticsearch integration has two repository indexers:
Maintaining two indexers is additional maintenance work, especially with large changes like gitaly#760 (closed) in the works.
We should commit to removing the Ruby elasticsearch indexer. The Go one has been around for over a year, and it has successfully indexed a significant proportion of projects on GitLab.com in the past with only a few bugs showing up: https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer/issues
We can resolve those issues before or after making the Go indexer the only choice; I don't have a strong opinion.