DevTools: Remove unused variables. Disallow unused variables with eslint

third_party/WebKit/Source/devtools/front_end/network/NetworkLogViewColumns.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 /**
  * @unrestricted
  */
 Network.NetworkLogViewColumns = class {
   /**
    * @param {!Network.NetworkLogView} networkLogView
    * @param {!Network.NetworkTransferTimeCalculator} timeCalculator
@@ skipping 452 matching lines @@
 
   /**
    * @param {string} headerId
    * @return {boolean}
    */
   _removeCustomHeader(headerId) {
     headerId = headerId.toLowerCase();
     var index = this._columns.findIndex(columnConfig => columnConfig.id === headerId);
     if (index === -1)
       return false;
-    var columnConfig = this._columns.splice(index, 1);
+    this._columns.splice(index, 1);
     this._dataGrid.removeColumn(headerId);
     this._saveColumns();
     this._updateColumns();
     return true;
   }
 
   /**
    * @param {string} headerTitle
    * @param {string=} headerId
    * @param {number=} index
@@ skipping 333 matching lines @@
 /**
  * @enum {string}
  */
 Network.NetworkLogViewColumns.WaterfallSortIds = {
   StartTime: 'startTime',
   ResponseTime: 'responseReceivedTime',
   EndTime: 'endTime',
   Duration: 'duration',
   Latency: 'latency'
 };