Wrong parameters for RPC function init

According to the HomeMatic XML RPC specification, to unregister a server, "interface_id" needs to be an empty string, not "url". I currently don't have a CCU to test this. So please check, if the change works. Also see: https://forum.homegear.eu/viewtopic.php?f=6&t=2&p=114#p105 and http://www.eq-3.de/Downloads/Software/HM-CCU2-Firmware_Updates/HM-CCU2-2.7.14/HM_XmlRpc_API-V2.100.pdf page 11.