Description of problem:
On a /reserveworkflow/ page, there is a scroll-down menu containing
a distro. It happens that distributions are removed from
lab-controlers or whatever, but they remain in that scroll-down
menu until manual removal.

Non functional distro results in 

    'No Systems compatible for that distro'

error on a beaker webui.

Does it make a sense, to remove the item from scroll-down menu,
when the item is removed from the last scheduler/controler?
In other words: would it be possible to do it automatically?