chg: nil: 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.

Merge branch 'ondrej/additional-cleanups-around-NSEC-namespace' into 'main'

See merge request isc-projects/bind9!11695