Clean up use to keep rust 1.43.0 from complaining (bp #9740) (#9748)

* Clean up `use` to keep rust 1.43.0 from complaining (#9740)

(cherry picked from commit c11abf88b7)

upload-perf/src/upload-perf.rs

@@ -1,4 +1,3 @@
-use serde_json;
 use serde_json::Value;
 use std::collections::HashMap;
 use std::env;