Issue Summary

When users delete a process step from a value stream, this causes work items associated with this process step to disappear from the backlog view process step view. This is confusing for customers due to the work item count at the top left not reflecting the number of cards/work items on this view. 

Cards that are associated with a deleted process step should be moved to the unassigned section. 

Steps to Reproduce

1. Step 1 - Navigate to the backlog view
2. Step 2 - Navigate to the process step view
3. If there isn't a workitem create one and assign it to a developmental process step
4. Then refresh the page to confirm the work item moved
5. Navigate to the value stream view and deleted the process step you assigned the work item to 
6. Then navigate to backlog process step view to see if you see the work item

Expected Results

The work item should show up under the unassigned section (This was approved by PM kforeman@atlassian.com

Actual Results

The work item disappears from this view

Workaround

No work around at this time