title: Why does Django South require a default value when removing a field?
body: <p>When I am removing a NOT NULL field using Django South, I get the following message:</p>
  
  <pre>
  ? The field 'VisitOrder.end_date' does not have a default specified, yet is NOT NULL.
  ? Since you are removing this field, you MUST specify a default
  ? value to use for existing rows. Would you like to:
  ?  1. Quit now, and add a default to the field in models.py
  ?  2. Specify a one-off value to use for existing columns now
  ?  3. Disable the backwards migration by raising an exception.
  ? Please select a choice: 
  </pre>
  
  <p>Why does South need this, given it's going to remove the field anyhow?</p>