DOC: Hello3DWorld remove setting cell

Per #466

Author: thewtex

docs/files/Hello3DWorld.ipynb

@@ -86,16 +86,6 @@
       ],
       "id": "26582452"
     },
-    {
-      "cell_type": "code",
-      "source": "viewer.set_image_gradient_opacity(0.4)",
-      "metadata": {
-        "trusted": true
-      },
-      "execution_count": 7,
-      "outputs": [],
-      "id": "5ffa3b37-ae13-4c5d-93e1-64d36c99b196"
-    },
     {
       "cell_type": "code",
       "source": "",
@@ -105,4 +95,4 @@
       "id": "9ac0de0c-7693-4a72-871b-0f7a9c4b2739"
     }
   ]
-}
+}