Remove unused import of BTreeMap in hash_table.rs

Author: kosiew

datafusion/physical-plan/src/aggregates/topk/hash_table.rs

@@ -416,7 +416,6 @@ mod tests {
     use super::*;
     use arrow::array::TimestampMillisecondArray;
     use arrow_schema::TimeUnit;
-    use std::collections::BTreeMap;
 
     #[test]
     fn should_emit_correct_type() -> Result<()> {