refactor(paroxysm): Use derive macros from serde crate

serde_derive is deprecated, the macros have moved behind a
feature-gate in serde itself.

Change-Id: Ib42e7463a81006e23a4fe8a0e48f494610dc8e4c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1550
Tested-by: BuildkiteCI
Reviewed-by: kanepyork <rikingcoding@gmail.com>

fun/paroxysm/src/cfg.rs

@@ -1,5 +1,5 @@
 use std::collections::HashSet;
-use serde_derive::Deserialize; // TODO(tazjin): move away from serde_derive
+use serde::Deserialize;
 
 #[derive(Deserialize)]
 pub struct Config {