fix_deleted_users.php doesn't actually work

The script admin/cli/fix_deleted_users.php claims to clean up users where someone has manually set "deleted = 1" through SQL instead of clicking the actual Delete button.

However, when actually run, it produces an exception because delete_user() -> context_user::instance() has an explicit check for "deleted = 0", so it's unable to find the user ID.

We use auth_saml2, so users are automatically created but there's no way to also have them accordingly deleted and suspended. So I use a custom tool to directly set suspended=1 for locked users, deleted=1 for users who no longer exist, and so on.

Now in my initial tests this appeared to work because I had visited the test user's profile page immediately before "deleting" it, so the context was still cached.

But when I actually wanted to use this method to bulk delete users which hadn't been active for many years (so they are not in cache), the problem is revealed: 
 

== Looking for sloppy user deletes ==
Redeleting user 3968: testuser1 (xxx@yyy.zzz)
Default exception handler: Invalid user Debug: SELECT id FROM {user} WHERE id = ? AND deleted = ?
[array (
{{ 0 => '3968',}}
{{ 1 => 0,}}
)]
Error code: invaliduser
* line 1646 of /lib/dml/moodle_database.php: dml_missing_record_exception thrown
* line 1622 of /lib/dml/moodle_database.php: call to moodle_database->get_record_select()
* line 6558 of /lib/accesslib.php: call to moodle_database->get_record()
* line 4249 of /lib/moodlelib.php: call to context_user::instance()
* line 74 of /admin/cli/fix_deleted_users.php: call to delete_user()!!! Invalid user !!!