fix(kms): drop stale KmsClient trait import in local_export tests

The KmsClient trait was folded into KmsBackend (#5501), but the backup
export tests merged afterwards (#5499) still imported it, breaking the
crate's test build; create_key is an inherent LocalKmsClient method, so
the import is simply unused.

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;