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>

virtio-devices/src/vhost_user/mod.rs

@@ -1,12 +1,6 @@
 // Copyright 2019 Intel Corporation. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
-extern crate epoll;
-extern crate net_util;
-extern crate vhost;
-extern crate virtio_bindings;
-extern crate vm_memory;
-
 use std::io;
 use vhost::Error as VhostError;
 use vm_memory::Error as MmapError;