Remove support for waterway=riverbank (#1405)

This PR removes support for `waterway=riverbank`.  Usage of waterway=riverbank has [declined to globally insignificant levels](https://taginfo.openstreetmap.org/tags/waterway=riverbank).

In other projects:
* Support for `waterway=riverbank` was removed from the iD editor in openstreetmap/iD#5591
* Removal of `waterway=riverbank` was tracked for JOSM in [JOSM ticket 21630](https://josm.openstreetmap.de/ticket/21630) and was completed in revision [18519](https://josm.openstreetmap.de/changeset/18519/josm)
* Removal of `waterway=riverbank` from openltmap (Lithuania vector map) occurred in [[1]](507f095b35)

layers/water/water.sql

@@ -1,7 +1,6 @@
 CREATE OR REPLACE FUNCTION water_class(waterway text, water text, leisure text) RETURNS text AS
 $$
 SELECT CASE
-           WHEN waterway='riverbank' THEN 'river'
            %%FIELD_MAPPING: class %%
            ELSE 'lake'
            END;