Remove mentions of jars from the Book

[mvaled]

The Salsa Book's tutorial includes (empty) mentions to #[salsa::jar] which was seemingly removed (see commit 65118a0).

There a many mentions of jars in the book which makes no sense when trying to read through the examples, e.g #[salsa::db(Jar1, ... JarN)] however this is never used in any code.

Similarly the Book mentions HasJars and HasJarsDyn; which are nowhere to be found in the code anymore (commit c84bd1b)

This is kind of confusing because the Tutorial starts with there jars without any salsa inside ;)