Clear out some unused code in QStorageIterator::next() on Unix

The code to save parent_id, actually a member variable that's
commented out, presumably because we have no use for it, was declaring
a variable to save it in and then Q_UNUSED()ing it. It is simpler to
just not do any of that. Since this "code" did at least document where
to get the parent_id, if we ever do want it, and what the number just
parsed actually means, keep the assignment as a comment but make clear
why it's commented out.

Change-Id: Iec59fa4d1af6c7e3b16570a612178a54004ebb70
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/io/qstorageinfo_unix.cpp

@@ -463,8 +463,7 @@ inline bool QStorageIterator::next()
     r = qstrntoll(ptr, stop - ptr, 10);
     if (!r.ok())
         return false;
-    int parent_id = r.result;
-    Q_UNUSED(parent_id);
+    // parent_id = r.result; // member currently not in use
 
     ptr += r.used;
     r = qstrntoll(ptr, stop - ptr, 10);