Issue 4580 - Replace ext.devtools with devtools

ext/background.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 59 matching lines @@
       let index = this._keys.indexOf(page._source);
       if (index >= 0)
       {
         this._keys.splice(index, 1);
         this._values.splice(index, 1);
       }
     }
   };
 
   window.ext.PageMap = PageMap;
-
-  window.ext.devtools = {
-    onCreated: {
-      addListener(listener)
-      {
-        window.addEventListener("message", (event) =>
-        {
-          if (event.data.type == "devtools")
-            listener(new ext.Page(event.source));
-        });
-      }
-    }
-  };
 }());