@Override
    public void process(boolean validate, boolean documentsOnly) {
        for (SDField field : schema.allConcreteFields()) {
            if (field.getRanking().isFilter()) {
                field.setRankType(RankType.EMPTY);
            }
        }
    }
All rank: filter fields should have rank type empty.

@author bratseth
config-model/src/main/java/com/yahoo/schema/processing/SetRankTypeEmptyOnFilters.java