drop default on system.leftline

[rettinghaus]

fixes #649

[kepper]

There's nothing wrong with this PR, so no worries. Also, this is more a reminder to myself than a complaint. However, I think we agreed at some point that such PRs can be commented and prepared here at any time, but that ultimately we wanted to have a short discussion during our ODD Fridays / Thursdays to take final decisions, so that these decisions become a little more transparent and less individual. We've never fully established that, and may need to think about how to implement that properly (perhaps we could use labels like "needs decision", or Milestones, but we shouldn't create too much overhead). Again, nothing wrong here, we just need to think about how to do this properly…