Cleanup the duplicate logic and comments around add into NSEC tree

Cleanup the duplicate logic and comments around add into NSEC tree

After merging the NORMAL, NSEC and NSEC3 tree into single QP tree, there
were some comments still speaking about auxiliary NSEC tree.  These were
cleaned up and the logic when we pass the qp tree (write transaction) to
qpzone_addrdataset_inner() was changed to be more obvious that this is
needed only when we are adding NSEC records.