remove old workaround of a Chrome/V8 bug

see
* https://github.com/openstreetmap/iD/issues/3973#issuecomment-309637397
* https://bugs.chromium.org/p/chromium/issues/detail?id=731193&can=1&q=prototype%20inheritance&colspec=ID%20Pri%20M%20Stars%20ReleaseBlock%20Component%20Status%20Owner%20Summary%20OS%20Modified

modules/core/graph.js

@@ -34,11 +34,6 @@ coreGraph.prototype = {
     entity: function(id) {
         var entity = this.entities[id];
 
-        //https://github.com/openstreetmap/iD/issues/3973#issuecomment-307052376
-        if (!entity) {
-            entity = this.entities.__proto__[id];  // eslint-disable-line no-proto
-        }
-
         if (!entity) {
             throw new Error('entity ' + id + ' not found');
         }