refactor: clean up some code in `QueryExt::select_ids` (#5520)
1. There was no need to use `drain` since we own the vector itself and
not a mutable reference to it, just `for row in rows` would have been
fine.
2. Rather than creating a second vector and using a loop to push the
transformed values, using `map` is more idiomatic (and more efficient in
this case, too, since the vec was created with `Vec::new` instead of
`Vec::with_capacity`).
3. The TODO comment seemed outdated, the `Select` passed to this
function already has the `traceparent` applied externally by the caller
(see `filter_ids`).