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/mapping.yaml

@@ -101,7 +101,6 @@ tables:
       - bay
       - spring
       waterway:
-      - riverbank
       - dock
       water:
       - river

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;

layers/water/water.yaml

@@ -19,8 +19,7 @@ layer:
     class:
       description: |
           All water polygons from [OpenStreetMapData](http://osmdata.openstreetmap.de/) have the class `ocean`.
-          Water bodies with the [`waterway=riverbank`](http://wiki.openstreetmap.org/wiki/Tag:waterway=riverbank)
-          or [`water=river`](http://wiki.openstreetmap.org/wiki/Tag:water=river) tag are classified as river.  Wet and dry docks
+          Water bodies with the [`water=river`](http://wiki.openstreetmap.org/wiki/Tag:water=river) tag are classified as river.  Wet and dry docks
           tagged [`waterway=dock`](http://wiki.openstreetmap.org/wiki/Tag:waterway=dock) are classified as a `dock`. 
           Swimming pools tagged [`leisure=swimming_pool`](https://wiki.openstreetmap.org/wiki/Tag:leisure=swimming_pool) are classified as a `swimming_pool`
           All other water bodies are classified as `lake`.
@@ -29,7 +28,6 @@ layer:
           waterway: 'dock'
         river:
           water: 'river'
-          waterway: 'riverbank'
         lake:
         ocean:
         swimming_pool: