Remove (Language) ResultSet.getResultDescription() method

(Language) ResultSet.getResultDescription() returns the ResultDescription of the top-level tree (ie. of the results that will be passed to the application) and thus is an attribute of the plan or activation. The ResultDescription is already available through the plan and activation.

Having this method of the ResultSet can lead to the assumption that it is the ResultDescription for that specific ResultSet, e.g. the shape of the results before any joins or projection. Note the main implementation of this method just calls activation.getResultDescription.