tracing: remove stale TODO

This TODO implied that orphaned spans (spans whose parent is
Finish()ing) are not made part of the active spans registry. But they
are, just above the TODO.

Release note: None

pkg/util/tracing/crdbspan.go

@@ -178,16 +178,13 @@ func (s *crdbSpan) finish() bool {
 		parent.childFinished(s)
 	}
 
+	// Deal with the orphaned children - make them roots.
 	for _, c := range children {
 		c.parentFinished()
 	}
-
 	// Atomically replace s in the registry with all of its still-open children.
 	s.tracer.activeSpansRegistry.swap(s.spanID, children)
 
-	// TODO(andrei): All the children that are still open are getting orphaned by
-	// the finishing of this parent. We should make them all root spans and
-	// register them with the active spans registry.
 	return true
 }