Removed the 'Gateway Domain' function from the entire Citadel system. This function...

Removed the 'Gateway Domain' function from the entire Citadel system.  This function has long since been superseded by the Global Address Book.  Absolutely no one is using it.  In fact, as far as I can tell, its implementation does not even exist anymore -- I couldn't find the back end code to remove.  So instead of confusing site administrators with a superfluous configuration option, we are removing it.