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>

performance-metrics/src/performance_tests.rs

@@ -8,7 +8,6 @@
 use crate::{mean, PerformanceTestControl};
 use std::fs;
 use std::path::PathBuf;
-use std::string::String;
 use std::thread;
 use std::time::Duration;
 use test_infra::Error as InfraError;