Remove extra farm OSM ids so we have one per farm, de-duplicated by proximity

[edwardchalstrey1]

• [x] Delete extra farm parts via the following steps:
  1. Find all farm object neighbours within 300m, not just the nearest
  2. SQL query: is osm_id int > neighbour id int, giving us a bool column which is true or false for each row of id pairs - call it "ordered" - into new table
  3. Select from this new table using func bool-and(ordered) and GROUP BY osm_id which will only give True for one id in each group, thereby selecting the osm_ids to retain - delete the others