Make specialist_sector_cleanup non-interactive

https://www.pivotaltracker.com/story/show/83309174

So that we can run the task easily using Fabric, we need to remove the interactivity from the `specialist_sector_cleanup` Rake task.

The task performs only one operation, so the removal of interactivity is not a significant functionality regression.

I've also added some additional output in cases where the sector slug is not provided.