Cannot create new super admins

[bramley]

This recent change disallow non-Superusers to manage admins
stops the creation of new super admins. The problem was raised on the support forum https://discuss.phplist.org/t/multiple-super-admins-no-longer-possible/9303

The setting of variable $accesslevel was removed but that is used later in the code to decide whether to display the Yes/No select list for whether the new admin is a super admin.

[michield]

This is now resolved