BTreeSet: remove duplicated code by reusing from_sorted_iter

The method `BTreeSet::from_sorted_iter` was introduced in 49ccb7519f,
but it was not consistently used throughout the codebase. As a result, some code redundantly reimplemented its logic.
This commit fixes the problem.

library/alloc/src/collections/btree/set.rs

@@ -1517,9 +1517,7 @@ fn from_sorted_iter<I: Iterator<Item = T>>(iter: I, alloc: A) -> BTreeSet<T, A>
 
         // use stable sort to preserve the insertion order.
         arr.sort();
-        let iter = IntoIterator::into_iter(arr).map(|k| (k, SetValZST::default()));
-        let map = BTreeMap::bulk_build_from_sorted_iter(iter, Global);
-        BTreeSet { map }
+        BTreeSet::from_sorted_iter(IntoIterator::into_iter(arr), Global)
     }
 }