So being the tidy person I am I tried to remove them, or rather set them to a "$null" value as follows:
I was met wtih an error message which I don't have at the moment. I later found out that this is a bug in both the RTM and SP1 builds of Exchange 2010. The bug has been pushed off to a post SP1 fix sometime later this year.
If you happen to set these values by accident, the guidance in the mean time is to make them equal to the primary FSW. In other words, make these values the same as the regular WitnessServer and WitnessDirectory entries.
In Exchange 2010 SP1 you can set these values but only when the DAG is configured in DAC mode.