Remove unnecessary ramp tagging (#1266)

This PR removes unnecessary `ramp` tagging from the `transportation` layer.  The `ramp` tag is used for two situations.  First, it is set for all `highway=***_link` tags to indicate a highway ramp.  The second situation is when the `ramp` tag is set on a highway object, which indicates the presence of a ramp, such as one that would be used for a wheelchair.  `ramp=0` is a reasonable default in these situations; by removing cases of `ramp=0` and presenting only `ramp=1` to the tiles, we can save considerable size in the tiles.  Additionally, there appears to be a bug in which all objects tagged `highway=steps` are inexplicably tagged `ramp=1`.

The changes as follows:

1. Remove `ramp=0` where it appears in the tile.
2. `highway=steps` no longer assumes `ramp=1` in cases where `ramp=*` is not explicitly set.

In the current behavior, `ramp=0` is tagged only in at z13+ but suppressed in lower zooms:
![image](https://user-images.githubusercontent.com/3254090/137589213-5a3432df-8c19-47bf-b556-7be15479790c.png)
![image](https://user-images.githubusercontent.com/3254090/137589296-f3126264-fb07-4346-ba70-c1f1b007a709.png)
![image](https://user-images.githubusercontent.com/3254090/137589306-30f9821b-3160-4b45-98b3-f2b8a0ae3a13.png)

This PR unifies `ramp=0` suppression between z13+ and z12-.

layers/transportation/transportation.sql

@@ -53,9 +53,9 @@ SELECT osm_id,
        NULLIF(network, '') AS network,
        -- All links are considered as ramps as well
        CASE
-           WHEN highway_is_link(highway) OR highway = 'steps'
-               THEN 1
-           ELSE is_ramp::int END AS ramp,
+           WHEN highway_is_link(highway)
+             OR is_ramp
+               THEN 1 END AS ramp,
        is_oneway::int AS oneway,
        brunnel(is_bridge, is_tunnel, is_ford) AS brunnel,
        NULLIF(service, '') AS service,