Remove not yet introduced stdin

[cknuepfer]

In the episode this solution is shown before the handling of the standard input. It is confusing to have file("stdin") at this point.

[zkamvar]

I'm helping the current lesson Maintainers process outstanding pull requests on this repository, in preparation for transition to the new lesson infrastructure.

Thank you for the contribution @cknuepfer! Looking at where this is in the lesson and the fact that it is only used once, I agree with your assessment and will merge this.