This change comes out of a discussion between Ben Kochie and me, around
this MR: https://gitlab.com/gitlab-org/gitlab-pages/merge_requests/164

gitlab-elasticsearch-indexer already uses `go mod` without a `vendor/`
directory. It has caused some intermittent build failures in the
gitlab-ce/ee CI pipelines, but has otherwise been fine. I think that
treating our Go dependencies in the same way we do our Ruby or Node.js
ones is reasonable, and it has some minor benefits:

* Contributors find it easier to submit MRs
* MRs are easier to review
* Makefiles are easier to write