remove unused StatGraph class

app/assets/javascripts/graphs/graphs_bundle.js

 require('./stat_graph_contributors_graph');
 require('./stat_graph_contributors_util');
 require('./stat_graph_contributors');
-require('./stat_graph');

app/assets/javascripts/graphs/stat_graph.js

-/* eslint-disable func-names, space-before-function-paren, wrap-iife, no-return-assign, max-len */
-(function() {
-  this.StatGraph = (function() {
-    function StatGraph() {}
-
-    StatGraph.log = {};
-
-    StatGraph.get_log = function() {
-      return this.log;
-    };
-
-    StatGraph.set_log = function(data) {
-      return this.log = data;
-    };
-
-    return StatGraph;
-  })();
-}).call(window);

spec/javascripts/graphs/stat_graph_spec.js

-/* eslint-disable quotes */
-/* global StatGraph */
-
-require('~/graphs/stat_graph');
-
-describe("StatGraph", function () {
-  describe("#get_log", function () {
-    it("returns log", function () {
-      StatGraph.log = "test";
-      expect(StatGraph.get_log()).toBe("test");
-    });
-  });
-
-  describe("#set_log", function () {
-    it("sets the log", function () {
-      StatGraph.set_log("test");
-      expect(StatGraph.log).toBe("test");
-    });
-  });
-});