Issue 6855 - Release all references to Subscription object once removed

lib/subscriptionClasses.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 134 matching lines @@
     if (this._disabled)
       buffer.push("disabled=true");
   },
 
   serializeFilters(buffer)
   {
     for (let filter of this.filters)
       buffer.push(filter.text.replace(/\[/g, "\\["));
   },
 
+  /**
+   * Releases any internal references to the object.
+   */
+  free()
+  {
+    Subscription.knownSubscriptions.delete(this.url);
+  },
+
   toString()
   {
     let buffer = [];
     this.serialize(buffer);
     return buffer.join("\n");
   }
 };
 
 /**
  * Cache for known filter subscriptions, maps URL to subscription objects.
@@ skipping 411 matching lines @@
     if (this.errors)
       buffer.push("errors=" + this.errors);
     if (this.version)
       buffer.push("version=" + this.version);
     if (this.requiredVersion)
       buffer.push("requiredVersion=" + this.requiredVersion);
     if (this.downloadCount)
       buffer.push("downloadCount=" + this.downloadCount);
   }
 });