The $reciprocal property in Fieldmanager_Autocomplete has no purpose

`Fieldmanager_Autocomplete` looks to have a [`$reciprocal` property](https://github.com/alleyinteractive/wordpress-fieldmanager/blob/861ba6d2d8301f36374231565a68a698136e4b0f/php/class-fieldmanager-autocomplete.php#L31-L36), but as best I can tell, it serves no purpose. That said, if someone's code is setting it, even though it does nothing, removing it will trigger an exception. We should think through how best to address this.