Revert "Autologin: Test logged session against remote user"

This reverts commit 64954e9924.

If the strip_username_regex is configured on the autologin backend and applies on a user's name,
the authenticated user's username does never match the REMOTE_USER server variable.
Thus the application will logout/login on every request which results in a redirect loop.

refs #6462

library/Icinga/Authentication/Manager.php

@@ -117,13 +117,6 @@ class Manager
     public function authenticateFromSession()
     {
         $this->user = Session::getSession()->get('user');
-
-        if ($this->user
-            && array_key_exists('REMOTE_USER', $_SERVER)
-            && $this->user->getUsername() !== $_SERVER['REMOTE_USER']
-        ) {
-            $this->removeAuthorization();
-        }
     }
 
     /**