Disassociate a Shared Custom Field from an object type

Remove the association between a shared custom field and an object type. The field will immediately disappear from API responses for that object type and all stored values will be permanently deleted. The `object_type` in the URL can be `lead`, `contact`, `opportunity`, `custom_activity_type/<catype_id>`, or `custom_object_type/<cotype_id>`. If the field was `required` on the associated type, removing the association also removes that requirement.