MDL-71099 Lib: Move new user_fields class from core to core_user

This class would belong more appropriately within the 'user' API
(core_user) instead of within the 'core' API, since it is
directly related to user data.

Since the class has only just been added to Moodle, now is a good
time to move it.

admin/classes/task_log_table.php

@@ -125,7 +125,7 @@ class task_log_table extends \table_sql {
         }
 
         // TODO Does not support custom user profile fields (MDL-70456).
-        $userfieldsapi = \core\user_fields::for_identity(\context_system::instance(), false)->with_userpic();
+        $userfieldsapi = \core_user\fields::for_identity(\context_system::instance(), false)->with_userpic();
         $userfields = $userfieldsapi->get_sql('u', false, 'user', 'userid2', false)->selects;
 
         $where = '';