remove empty optional element

- required for kodi addon-check

Author: anxdpanic

addon.xml

@@ -18,7 +18,6 @@
         <summary lang="en_GB">Module for interaction with the Twitch.tv API</summary>
         <description lang="en_GB">python-twitch for Kodi is module for interaction with the Twitch.tv API based on python-twitch by ingwinlu.</description>
         <license>GNU GENERAL PUBLIC LICENSE. Version 3, 29 June 2007</license>
-        <forum> </forum>
         <source>https://github.com/MrSprigster/script.module.python.twitch</source>
         <reuselanguageinvoker>true</reuselanguageinvoker>
     </extension>