- removed slash-adding for logout-header-redirect

this slash is not necessary for redirecting. 
-> with the removal your have the chance to manipulate OC::$WEBROOT for a different logout-redirection

possible usecases:
- log the site your are coming from(directly)
- write a logout-hook and manipulate webroot to the site your are coming from
- logout and get redirected to the site you're coming from..