Fix compatibility with Moodle 4

[azmatksa]

In Moode 4, it shows an error, there is a field hidden that's not exist in question table while in query it's using.

[Shotte]

@azmatksa
I had to do the following, because a database column named "hidden" for table "mdl_question" no longer exists:

1. go to the file location <...>/moodle/report/questionstats/
2. edit file index.php
3. delete from line 50 and 53 (SQL queries) the text "Q.hidden = 0 AND"
4. save file and it should work

If you addionally want to solve that a couple of questions are announced as "Unknown plugin type", change lines 78 to 80 as follows:

if ($ctype_name == '[[pluginname]]') {
$ctype_name = '[' . $item->typename . ']';
}
else {
$ctype_name = $ctype_name . ' [' . $item->typename . ']';
}

Now it looks all fine like this: