Implements ArrayAccess::offsetUnset().
public function offsetUnset($offset) {
unset($this->definition[$offset]);
// If the widget or formatter properties changed, the corrsponding plugins
// need to be re-instanciated.
if ($offset == 'widget') {
unset($this->widget);
}
if ($offset == 'display') {
unset($this->formatters);
}
}