Drop respdiff-short CI jobs

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
f09cf69594c6aab4d0c5608226424c566b833f3c) are no longer an issue.