BUGFIX: removing reliance on Parent::getCMSFields 

Removing reliance on the parent::getCMSFields within the Member::getCMSFields method for the following reasons:

- in line with the way CMS Fields are added by other core model classes such as Folder, File, and Group, none of which use parent::getCMSFields.

- more efficient (not adding a whole bunch of fields and then removing them again)

- updateCMSFields extension called only once

- Less intimidating Member Fields with the less commonly used fields out of the way in separate tab (locale settings).

Original ticket: https://github.com/silverstripe/silverstripe-framework/issues/2827