Revert #6287 for graph_tooltip.js

public/app/plugins/panel/graph/graph_tooltip.js

@@ -21,10 +21,7 @@ function ($) {
       var initial = last*ps;
       var len = series.datapoints.points.length;
       for (var j = initial; j < len; j += ps) {
-        // Special case of a non stepped line, highlight the very last point just before a null point
-        if ((series.datapoints.points[initial] != null && series.datapoints.points[j] == null && ! series.lines.steps)
-            //normal case
-            || series.datapoints.points[j] > posX) {
+        if (series.datapoints.points[j] > posX) {
           return Math.max(j - ps,  0)/ps;
         }
       }