misc: Remove redundant "use" imports

With the nightly toolchain (2024-02-18) cargo check will flag up
redundant imports either because they are pulled in by the prelude on
earlier match.

Remove those redundant imports.

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

devices/src/legacy/serial.rs

@@ -345,8 +345,7 @@ impl Migratable for Serial {}
 #[cfg(test)]
 mod tests {
     use super::*;
-    use std::io;
-    use std::sync::{Arc, Mutex};
+    use std::sync::Mutex;
     use vm_device::interrupt::{InterruptIndex, InterruptSourceConfig};
     use vmm_sys_util::eventfd::EventFd;