Stop building v2020dev0 images daily

The v2020dev0 release has been out for a while, all the fixes targeting it have been landed and we're now focusing on v2020pre, so there's no need to build it on a daily basis.

Builds can be triggered by hand when needed.

See also https://gitlab.apertis.org/infrastructure/apertis-internal-jenkins-jobs/merge_requests/8