src: remove `StreamResourc::Cast()`

This is not necessary since C++ already has `static_cast`
as a proper way to cast inside a class hierarchy.

PR-URL: https://github.com/nodejs/node/pull/17564


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>