Remove Jenkins jobs that are now covered by GitLab CI

We can do this now that our GitLab CI setup is in production :)

After merging this, please remove the inclusion of tails::website::builder on isobuilders (modules/local/manifests/node/isobuilder.pp).

Reminder for the reviewer: this GitLab project is a read-only mirror.

Closes sysadmin#17776 (closed)