Issue 6855 - Release all references to Subscription object once removed

lib/synchronizer.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 186 matching lines @@
     {
       let oldSubscription = Subscription.fromURL(downloadable.url);
       subscription.title = oldSubscription.title;
       subscription.disabled = oldSubscription.disabled;
       subscription.lastCheck = oldSubscription.lastCheck;
 
       let listed = FilterStorage.knownSubscriptions.has(oldSubscription.url);
       if (listed)
         FilterStorage.removeSubscription(oldSubscription);
 
-      Subscription.knownSubscriptions.delete(oldSubscription.url);
+      oldSubscription.free();
 
       if (listed)
         FilterStorage.addSubscription(subscription);
     }
 
     // The download actually succeeded
     subscription.lastSuccess = subscription.lastDownload = Math.round(
       Date.now() / MILLIS_IN_SECOND
     );
     subscription.downloadStatus = "synchronize_ok";
@@ skipping 130 matching lines @@
               subscription.filters.map(f => f.text).join("\n");
             redirectCallback("data:text/plain," + encodeURIComponent(data));
           }
         }, false);
         request.send(null);
       }
     }
   }
 };
 Synchronizer.init();