Fix goal plan specification deletion

[duranb]

Resolves #1551

To test:

1. Create two plans
2. Create a scheduling goal
3. Open one plan
4. Open the Scheduling Goals panel
5. Click Manage Goals
6. Select the created scheduling goal from step refactor: sim result activities property change #2
7. Click Update
8. Repeat steps 4-7 on the second plan
9. Open the Manage Goals modal on either of the plans
10. Deselect the goal
11. Click Update
12. Verify that the goal has been removed from the intended plan and is still present on the other plan