Croissant: fix semantic_seg shape (12,480,640) uint16, drop spurious channel dim

yonder.croissant.json

@@ -231,10 +231,10 @@
           "@type": "cr:Field",
           "@id": "waypoints/semantic_seg",
           "name": "semantic_seg",
-          "description": "Per-pixel semantic instance + class IDs. Shape (12, 480, 640, 2) int32. Available for all 167 HSSD scenes.",
+          "description": "Per-pixel semantic instance IDs. Shape (12, 480, 640) uint16, one channel per yaw. Available for all 167 HSSD scenes; stored as separate {scene}_wp{NNNN}_semantics.npz files under semantics/{scene}/ in the repo.",
           "dataType": "sc:Integer",
           "isArray": true,
-          "arrayShape": "12,480,640,2"
+          "arrayShape": "12,480,640"
         }
       ]
     }