Remove unused extern crates

deltachat-ffi/src/lib.rs

@@ -1,4 +1,4 @@
-#![deny(clippy::all)]
+#![deny(unused, clippy::all)]
 #![allow(
     non_camel_case_types,
     non_snake_case,
@@ -11,8 +11,6 @@
 
 #[macro_use]
 extern crate human_panic;
-extern crate num_traits;
-extern crate serde_json;
 
 use std::collections::BTreeMap;
 use std::convert::TryFrom;

src/lib.rs

@@ -2,6 +2,7 @@
 
 #![forbid(unsafe_code)]
 #![deny(
+    unused,
     clippy::correctness,
     missing_debug_implementations,
     clippy::all,
@@ -24,7 +25,6 @@ extern crate num_derive;
 extern crate smallvec;
 #[macro_use]
 extern crate rusqlite;
-extern crate strum;
 #[macro_use]
 extern crate strum_macros;