rm shape:'spline' from range slider reversed range mock

... which rendered oddly in our nw.js imagetest container.

Author: etpinard

test/image/baselines/range_slider_reversed-range.png

@@ -5,8 +5,7 @@
       "hoverinfo": "x",
       "showlegend": false,
       "line": {
-        "color": "blue",
-        "shape": "spline"
+        "color": "blue"
       },
       "x": [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11],
       "y": [0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120]