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

html/ajax_form.php

@@ -12,12 +12,10 @@
  * the source code distribution for details.
  */
 
-use LibreNMS\Authentication\LegacyAuth;
-
 $init_modules = array('web', 'auth', 'alerts');
 require realpath(__DIR__ . '/..') . '/includes/init.php';
 
-if (!LegacyAuth::check()) {
+if (!Auth::check()) {
     die('Unauthorized');
 }