fix lint issue

Author: antoinerg

src/traces/sankey/attributes.js

@@ -14,7 +14,6 @@ var colorAttrs = require('../../components/color/attributes');
 var fxAttrs = require('../../components/fx/attributes');
 var domainAttrs = require('../../plots/domain').attributes;
 
-var extendFlat = require('../../lib/extend').extendFlat;
 var overrideAll = require('../../plot_api/edit_types').overrideAll;
 
 var attrs = module.exports = overrideAll({