misc: Remove unnecessary "extern crate"

Now all crates use edition = "2018" then the majority of the "extern
crate" statements can be removed. Only those for importing macros need
to remain.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vm-migration/src/lib.rs

@@ -3,11 +3,8 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 //
 
-extern crate serde;
-extern crate thiserror;
 #[macro_use]
 extern crate serde_derive;
-extern crate vm_memory;
 
 use anyhow::anyhow;
 use serde::{Deserialize, Serialize};