fix(kms): drop the KmsClient trait import removed by #5501

The local backup export tests (#5499) merged after #5501 folded the
KmsClient trait into KmsBackend, leaving a dead trait import that breaks
'cargo test -p rustfs-kms' compilation on main. create_key is an inherent
method on LocalKmsClient since #5501, so the import is unnecessary.

crates/kms/src/backup/local_export.rs

@@ -560,7 +560,6 @@ async fn fsync_dir(path: &Path) -> Result<()> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::backends::KmsClient;
     use crate::config::LocalConfig;
     use std::sync::Arc;
     use tempfile::TempDir;