portindex.tcl: Drop support for specifying stdlib

[mojca]

While trying to fix https://trac.macports.org/ticket/59318 I only noticed a duplicate at https://trac.macports.org/ticket/55471 when I tried to make a pull request (I was initially working on the wrong/old/generated file).

Maybe we nevertheless want to drop support for specifying stdlib as it's unlikely we'll need it again. See: https://trac.macports.org/ticket/59069