Remove table building_multipolygon as multipolygon are already in table osm_building_polygon (#756)

Buildings from ways and multipolygons are loaded in table `osm_building_polygon`. But a table for `osm_building_multipolygon` is also loaded, the content is not used except to ensure an `osm_id` is from a multipolygon. To check if the object is from a multipolygon we have only to check if `osm_id` is negative. It is the counter part of e0c8ece375/layers/building/building.sql (L89)

I checked the objects are the same after this change.

layers/building/mapping.yaml

@@ -342,76 +342,3 @@ tables:
     mapping:
       type: [building]
     type: relation_member
-
-  # etldoc: imposm3 -> osm_building_multipolygon
-  building_multipolygon:
-    columns:
-    - name: osm_id
-      type: id
-    - name: geometry
-      type: validated_geometry
-    - name: area
-      type: area
-    - name: building
-      key: building
-      type: string
-      from_member: true
-    - name: material
-      key: building:material
-      type: string
-    - name: colour
-      key: building:colour
-      type: string
-    - name: buildingpart
-      key: building:part
-      type: string
-      from_member: true
-    - name: buildingheight
-      key: building:height
-      type: string
-      from_member: true
-    - name: height
-      key: height
-      type: string
-      from_member: true
-    - name: buildingmin_height
-      key: building:min_height
-      type: string
-      from_member: true
-    - name: min_height
-      key: min_height
-      type: string
-      from_member: true
-    - name: buildinglevels
-      key: building:levels
-      type: string
-      from_member: true
-    - name: levels
-      key: levels
-      type: string
-      from_member: true
-    - name: buildingmin_level
-      key: building:min_level
-      type: string
-      from_member: true
-    - name: min_level
-      key: min_level
-      type: string
-      from_member: true
-    - name: member
-      type: member_id
-    - name: index
-      type: member_index
-    - name: role
-      type: member_role
-      from_member: true
-    - name: type
-      type: member_type
-    mapping:
-      type: [multipolygon]
-    filters:
-      reject:
-        building: ["no","none","No"]
-        building:part: ["no","none","No"]
-        man_made: ["bridge"]
-    type: relation_member