Don't use extended_bounds for time filter in date_histogram vis

The date histogram vis currently uses extended_bounds on the date_histogram if a time filter is applied which means that these requests can't be cached since the cache key for the request cache for Elasticsearch is based on the request body. The `extended_bounds` here has no effect on the response of the date histogram aggregation because it also sets minDocCount to 0 so will not return any empty buckets.

This change removes the extended bounds parameter for the time filter so these requests can be cached by Elasticsearch.

src/ui/public/agg_types/buckets/date_histogram.js

@@ -150,14 +150,6 @@ export default function DateHistogramAggType(timefilter, config, Private) {
 
             return;
           }
-
-          var bounds = timefilter.getActiveBounds();
-          if (bounds) {
-            output.params.extended_bounds = {
-              min: moment(bounds.min).valueOf(),
-              max: moment(bounds.max).valueOf()
-            };
-          }
         }
       }
     ]