[es6] use elasticsearch default style for highlighting (<em> tag)

cubicweb_elasticsearch/views.py

@@ -77,9 +77,7 @@ class CWFacetedSearch(FacetedSearch):
         Add custom highlighting
         """
         search = search.highlight(*(f if '^' not in f else f.split('^', 1)[0] for f in self.fields))
-        return search.highlight_options(pre_tags="",
-                                        post_tags="",
-                                        fragment_size=150)
+        return search.highlight_options(fragment_size=150)
 
 
 class ElasticSearchView(StartupView):