expr exccopen: only pass SFIO stream to callback

expr exccopen: only pass SFIO stream to callback

This function invokes `dtwalk` passing the callback `global` to be called with
a state parameter it supplies. It was supplying `cc`, but the callback only uses
a single member of this struct. So we can simplify this code by just passing the
stream the callback writes to, `cc->ccdisc-text`, or equivalently `disc->text`.