BUGFIX: avoid calling updateCMSFields 2x in Member

```php
$this->extend('updateCMSFields', $fields); 
```

is called in DataObject::getCMSFields, but it is also called in Member::getCMSFields.  While it is important to have an extend in both, it is not so useful to call the same method twice.  Therefore, we call  updateMemberCMSFields in Member.

See https://github.com/silverstripe/silverstripe-framework/issues/2827