Remove standard io chapter from the book

This was originally used to set up the guessing game, but that no longer
exists. This version uses `old_io`, and updating it involves talking
about `&mut` and such, which we haven't covered yet. So, for now, let's
just remove it.

Fixes #23760

Author: steveklabnik

Diff for: src/doc/trpl/SUMMARY.md

@@ -13,7 +13,6 @@
     * [Looping](looping.md)
     * [Strings](strings.md)
     * [Arrays, Vectors, and Slices](arrays-vectors-and-slices.md)
-    * [Standard Input](standard-input.md)
 * [Intermediate Rust](intermediate.md)
     * [Crates and Modules](crates-and-modules.md)
     * [Testing](testing.md)