Two examples had a superfluous bound
': salsa::Database' that wasn't present
in the `compiler` example.
The `query_group` macro adds this bound
automatically.
This change can lead to a trailing `+` in
the bounds list. I verified this is OK by
running the examples and verifying that the production
is allowed
[per the Rust Reference](https://doc.rust-lang.org/reference/trait-bounds.html)