docs: public ExtensionElement removeExtension(ExtensionElement extension)  {
          String key = XmppStringUtils.generateKey(extension.getElementName(), extension.getNamespace());
          synchronized (packetExtensions) {
              List<ExtensionElement> list = packetExtensions.getAll(key);
              boolean removed = list.remove(extension);
              if (removed) {
                  return extension;
              }
          }
          return null;
      }