Don't store full blob path in ES filename field

This verifies the changed indexer behaviour from
https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer/merge_requests/44

We're also renaming the blob-related code to use `path` rather than
`filename`, to be consistent with the `Path` field in the ES index,
and the intention of this code (which deals with full paths, rather
than just filenames).

This also removes the `Elasticsearch::Git::LiteBlob` class, which isn't
used anymore and a leftover from the Ruby indexer which was removed in
https://gitlab.com/gitlab-org/gitlab/merge_requests/15641