Remove legacy auth usage of $_SESSION (#10491)

* Remove auth use of $_SESSION

Will break plugins that depend on $_SESSION, Weathermap was already fixed.
Port them to use Auth::check()/Auth::user()/Auth:id()

* revert accidental replacement

includes/html/forms/ack-alert.inc.php

@@ -23,7 +23,6 @@
  * @author     Neil Lathwood <gh+n@laf.io>
  */
 
-use LibreNMS\Authentication\LegacyAuth;
 use LibreNMS\Config;
 
 header('Content-type: application/json');
@@ -60,7 +59,7 @@ if (!is_numeric($alert_id)) {
         'until_clear' => $until_clear,
     ]);
 
-    $username = LegacyAuth::user()->username;
+    $username = Auth::user()->username;
     $data = [
         'state' => $state,
         'open' => $open,