Fix clippy lint: replace filter_map identity with flatten()

Author: EliteCoder18

tests/bnb.rs

@@ -221,7 +221,7 @@ fn bnb_tuple_metric_respects_ordering_requirement() {
     let cs = CoinSelector::new(&candidates);
     let solutions = cs.bnb_solutions(((OrderSensitive, 1.0), (NoOrderMetric, 1.0)));
 
-    let first_solution = solutions.filter_map(|sol| sol).next();
+    let first_solution = solutions.flatten().next();
     assert!(
         first_solution.is_some(),
         "expected a solution when ordering is required by any metric"