Remove some unused #[derive] (#473)

We used to need them, probably when we were using Tera.

plume-models/src/lib.rs

@@ -21,8 +21,6 @@ extern crate rocket;
 extern crate scheduled_thread_pool;
 extern crate serde;
 #[macro_use]
-extern crate serde_derive;
-#[macro_use]
 extern crate serde_json;
 #[macro_use]
 extern crate tantivy;