reworked landcover layer (#1035)

Instead of using [Douglas-Peucker algorithm](https://postgis.net/docs/ST_SimplifyPreserveTopology.html) which is using for generalized tables [imposm](https://imposm.org/docs/imposm3/latest/mapping.html#generalized-tables) it is used [Visvalingam-Whyatt algorithm](https://postgis.net/docs/ST_SimplifyVW.html).

Solution:
remove imposm generalized tables (during `import-osm`) and create generalization in `import-sql` step for zooms 7-14. Zooms 0-6 are from Natural Earth data.

Upper zoom levels (7 and 8) are ok to merge with no big impact on creation speed. In Canada from z9 it took too long. Between zoom levels 10 - 13 there can be union polygons with less than 300 edge points (empirical number based on a test on Canada forest, can be discussed). Polygons with more than 300 edge points are just simplified. Zoom 14 is from the original dataset `osm_landcover_polygon`.

There is also removed `osm_id` which is never used and not passed into vector tiles.

layers/landcover/mapping.yaml

@@ -1,54 +1,7 @@
-
-generalized_tables:
-
-  # etldoc: imposm3 -> osm_landcover_polygon_gen7
-  landcover_polygon_gen7:
-    source: landcover_polygon_gen6
-    sql_filter: area>power(ZRES5,2)
-    tolerance: ZRES7
-
-  # etldoc: imposm3 -> osm_landcover_polygon_gen6
-  landcover_polygon_gen6:
-    source: landcover_polygon_gen5
-    sql_filter: area>power(ZRES6,2)
-    tolerance: ZRES8
-
-  # etldoc: imposm3 -> osm_landcover_polygon_gen5
-  landcover_polygon_gen5:
-    source: landcover_polygon_gen4
-    sql_filter: area>power(ZRES7,2)
-    tolerance: ZRES9
-
-  # etldoc: imposm3 -> osm_landcover_polygon_gen4
-  landcover_polygon_gen4:
-    source: landcover_polygon_gen3
-    sql_filter: area>power(ZRES8,2)
-    tolerance: ZRES10
-
-  # etldoc: imposm3 -> osm_landcover_polygon_gen3
-  landcover_polygon_gen3:
-    source: landcover_polygon_gen2
-    sql_filter: area>power(ZRES8,2)
-    tolerance: ZRES11
-
-  # etldoc: imposm3 -> osm_landcover_polygon_gen2
-  landcover_polygon_gen2:
-    source: landcover_polygon_gen1
-    sql_filter: area>power(ZRES9,2)
-    tolerance: ZRES12
-
-  # etldoc: imposm3 -> osm_landcover_polygon_gen1
-  landcover_polygon_gen1:
-    source: landcover_polygon
-    sql_filter: area>power(ZRES10,2) AND ST_IsValid(geometry)
-    tolerance: ZRES13
-
 tables:
   # etldoc: imposm3 -> osm_landcover_polygon
   landcover_polygon:
     columns:
-    - name: osm_id
-      type: id
     - name: geometry
       type: validated_geometry
     - name: area