removeClasses remove observations corresponding to
  specified classes
 
    subDataSet = dataSet.removeClasses(classes) returns a
    data set subDataSet containing only the observations that
    have DO NOT have targets equal to the specied values
 
    classes can be specified as an array of class values,
    or as a cell array of strings containing the class names.
 
    ds = prtDataGenIris;
    ds12 = ds.removeClasses(3);
 
    ds12_v2 = ds.removeClasses({'Iris-virginica'});