Remove unused access to (Special)CheckUser object
This is an alternative for the workaround presented in Id2634a5. I
followed all code paths and could not find any usage of the
CheckUserRow::checkUser property I'm removing in this patch. Nor could
I find any usage of the CheckUserRow class.
The more general question is: Why does a FormatterRow need access to
the entirety of this specific special page?
Id2634a5 might still be valueable as it is easier to backport.