idoverride{user,group}: Fix delete_continue with state absent

All tasks for idoverrideuser and idoverridegroup with state absent
failed with "'continue' is required" when delete_continue was not set.

This happended as delete_continue was internally None and continue: None
was provided to the API.

The fix is simply to use '"continue": delete_continue or False' so that
continue is set to False in this case.