units: Remove serde re-export

We re-export to help users keep their dependencies in sync but `serde`
is at `v1.0` so this is not really a problem.

Remove the public re-export of `serde` (and with current MSRV we don't
need the `extern crate` at all).

units/src/lib.rs

@@ -27,10 +27,6 @@ extern crate alloc;
 #[cfg(feature = "std")]
 extern crate std;
 
-/// A generic serialization/deserialization framework.
-#[cfg(feature = "serde")]
-pub extern crate serde;
-
 pub mod amount;
 #[cfg(feature = "alloc")]
 pub mod block;