code: public TimeFormat removeUnit(final TimeUnit unit)
     {
        if (unit == null)
           return null;
  
        cachedUnits = null;
  
        return units.remove(unit);
     }
docs: Removes the mapping for the given {@link TimeUnit}. This effectively de-registers the {@link TimeUnit} so it will
  not be used in formatting. Returns the {@link TimeFormat} that was removed, or null if no such unit was
  registered.