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-virtio/src/queue.rs

@@ -966,8 +966,6 @@ pub mod testing {
 
 #[cfg(test)]
 pub mod tests {
-    extern crate vm_memory;
-
     use super::testing::*;
     pub use super::*;
     use vm_memory::{GuestAddress, GuestMemoryMmap};