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>

net_util/src/lib.rs

@@ -10,16 +10,8 @@
 #[cfg(test)]
 #[macro_use]
 extern crate lazy_static;
-extern crate libc;
 #[macro_use]
 extern crate log;
-extern crate net_gen;
-extern crate rate_limiter;
-extern crate serde;
-extern crate virtio_bindings;
-extern crate vm_memory;
-extern crate vm_virtio;
-extern crate vmm_sys_util;
 
 mod mac;
 mod open_tap;

net_util/src/mac.rs

@@ -152,8 +152,6 @@ impl FromStr for MacAddr {
 
 #[cfg(test)]
 mod tests {
-    extern crate serde_json;
-
     use super::*;
 
     #[test]