Remove ZERO_WIDTH_SPACE_STRING, relying on iOS 6 to do it right.

As of iOS 6, a UITextField subclass will receive -deleteBackward even when the text field is empty. So ZERO_WIDTH_SPACE_STRING is no longer needed there.

This also takes care of the rare case where the user dragged the insertion point back to the beginning of the field, *before* the zero-width space. That is no longer possible.