build: drop unused serde direct dependency

`serde` (the crate) was listed in [dependencies] with the `derive`
feature, but no code in this crate calls `use serde::*` or derives
`Serialize`/`Deserialize`. Only `serde_json` is used.

Removing it doesn't break any transitive dependency: pgwire and
trino-rust-client pull serde in themselves. `cargo build`,
`cargo test --lib`, and `cargo clippy --all-targets` all still pass.

Author: lfrancke

Cargo.lock

@@ -20,7 +20,6 @@ tracing-subscriber = "0.3"
 anyhow = "1"
 serde_json = "1"
 trino-rust-client = "0.9"
-serde = { version = "1", features = ["derive"] }
 async-stream = "0.3"
 dashmap = "6"
 sqlparser = { version = "0.61", features = ["visitor"] }