Fix workflow schema field references on tasks when removed/renamed

Steps to reproduce:
1. Create a workflow.
2. Add a workflow variable var1 (single-valued string).
3. Save.
4. In the graph, add an "Approve" node between the "Start" and "Stop" nodes.
5. Edit the new node and make var1 editable in this task.
6. Save.
7. Remove or rename the var1 variable.
8. Save.
9. Notice that there are no editable vars for the "Approve" node (acceptable behaviour when removing the var).
10. Re-add or rename (depending on what was done on step 7) the existing variable using the original name (var1) in the workflow variables.
11. Save.
12. Notice that var1 is still editable in the "Aprove" node.

This means that these references are not correctly maintained. This can be verified by downloading the studio bundle and examining the generated configuration. After removing or renaming workflow variables (used in task nodes), the original references are still laying around.

Fixing the deletion should be fairly simple, although figuring out if a variable is being renamed could be very tricky.