How to remove "Extended Demographics"?
The easiest would be if you can find it in the CiviCRM custom groups? Using Administer/Customize Data and Screens/Custom Fields. If that does not work, check the table civicrm_custom_group. You will probably see an entry there with the column name 'is_reserved' having the value 1. 
If that is the case, change the value for 'is_reserved' to 0 and check if you can see the custom group in the Administer menu now?

This would be the best way as you would delete all that you want to delete. If that fails, shout again?