MDL-38497 Fix invalid $ADMIN->add() calls

MDL-38497 Fix invalid $ADMIN->add() calls

As the admin_category::add() method now checks for the third parameter,
couple of typos were detected in the code. Additional parameters passed
to the add() method were probably a mistake, a relict or a typo.

Note that the typo in admin/tool/unsuproles/settings.php had actually
significant impact on the functionality as the array with required
capabilities was not actually passed to the admin_externalpage
constructor as intended.