Drop respdiff-short CI jobs

In the past, our CI infrastructure was more sensitive to the number of CI jobs running on it. We tried to limit long-running jobs in merge request-triggered pipelines, as there are many of them, and spawned them only in daily scheduled ones. Moving most of the CI infrastructure to AWS has made it way better to run jobs in parallel, and the existence of short respdiff jobs has lost its original merit. It can also be harmful as some problems are detected only by the longer respdiff variant when a faulty merge request has already been merged. We should run all long respdiff tests in merge request-triggered pipelines.

Also, move the former respdiff-long job (now just "respdiff") to AWS as old instance memory constraints (see f09cf695) are no longer an issue.