block: Fix clippy: unused import

```
warning: unused import: `Bytes`
   --> block/src/io/request.rs:659:21
    |
659 |     use vm_memory::{Bytes as _, GuestMemoryMmap};
    |                     ^^^^^^^^^
    |
    = note: `-D unused-imports` implied by `-D warnings`
```

Signed-off-by: Rob Bradford <rbradford@meta.com>

block/src/io/request.rs

@@ -660,7 +660,7 @@ impl Request {
 mod unit_tests {
     use std::sync::Arc;
 
-    use vm_memory::{Bytes as _, GuestMemoryMmap};
+    use vm_memory::GuestMemoryMmap;
     use vmm_sys_util::eventfd::EventFd;
 
     use super::*;