remove 'cliponaxis: false' from hist_stacked mock to test bar label clipping

Author: emilykl

test/image/mocks/hist_stacked.json

@@ -11,7 +11,6 @@
       "x": [1, 2, 3, 4],
       "text": "Orange",
       "textposition": "outside",
-      "cliponaxis": false,
       "texttemplate": "%{value}<br>%{text}",
       "type": "histogram"
     }