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>

rate_limiter/src/lib.rs

@@ -522,7 +522,6 @@ pub(crate) mod tests {
     use super::*;
     use std::fmt;
     use std::thread;
-    use std::time::Duration;
 
     impl TokenBucket {
         // Resets the token bucket: budget set to max capacity and last-updated set to now.