Object should be removed from Data Model dropdown after it's been deleted

[erinknight242]

When you define a new custom object, this object is added to the Data Model dropdown to be reused by future fields on that contract. However, when that model has been deleted from the contract, it still exists in the Options dropdown until the contract is saved and reopened.

Once a model no longer exists in a contract, it should be removed from the dropdown options list. This check should happen after deleting an object; include javascript tests to verify.