Remove unneeded waterway='' check (#1377)

This PR removes an unnecessary check in water.yml.

generated SQL before:
```
SELECT CASE
           WHEN waterway='riverbank' THEN 'river'
           WHEN "waterway" = 'dock' THEN 'dock'
           WHEN "water" = 'river'
               OR "waterway" = 'riverbank'
               THEN 'river'
           WHEN "waterway" = '' THEN 'lake'
           ELSE 'lake'
           END;
```

generated SQL after:
```
SELECT CASE
           WHEN waterway='riverbank' THEN 'river'
           WHEN "waterway" = 'dock' THEN 'dock'
           WHEN "water" = 'river'
               OR "waterway" = 'riverbank'
               THEN 'river'
           ELSE 'lake'
           END;
```

The `waterway=''` test is purely redundant because it returns the same value as the fall-through ELSE case: `lake`.

layers/water/water.yaml

@@ -26,7 +26,6 @@ layer:
           water: 'river'
           waterway: 'riverbank'
         lake:
-          waterway: ''
         ocean:
     intermittent:
       description: |