Update _set_axis(): fix layout property deletion bug

[rightx2]

fig['layout'] in _set_axis(), which is an object of plotly.graph_objs.Layout should not be used with del because the class and its all parent classes do not implement __delitem__. So, it will raise AttributeError: __delitem__ instead of KeyError.

So, I fixed it by accessing _props directly. But it might not look good because the underscore + property is usually called inside of an instance method, not the outside of the object. Any comments or fixes will be welcomed!