Fixed ezp-18520: eZ Flow cannot handle sso logins

when a SSO login handler is defined, if a user is found through the handler it redirects directly, the lines removed in this commit  are making a "short-cut" of the normal user processing. 

it's useless, and breaks the expected login process. the ezuser::instance()  is defined to return an user, a valid one, the anon, or a special nouser. all these cases are valid, there is no need for a hard redirect in the middle of the function to treat sso.

- breaks ezflow editing, causing logouts or the POSTs to save/publish a frontpage object.
- also breaks regular logins when extra data is passed by querystring or POST
- breaks the functionality of the instance() function, causing an unexpected jump.

if an user is found with the sso handler,  the code block should set the currentuser and let the function terminate normally, returning one user. this redirect is just a masked GOTO, and not even needed.