Fix Coverity CID 487884: Dead code in qpcache.c

Fix Coverity CID 487884: Dead code in qpcache.c

Adding a changed record is zonedb related and does not belong in
the cache code. This is a leftover dead code and can be safely
removed.

/lib/dns/qpcache.c: 3459 in add()
3453     			}
3454     			newheader->next = topheader->next;
3455     			newheader->down = topheader;
3456     			topheader->next = newheader;
3457     			qpnode->dirty = 1;
3458     			if (changed != NULL) {
>>>     CID 487884:    (DEADCODE)
>>>     Execution cannot reach this statement: "changed->dirty = true;".
3459     				changed->dirty = true;
3460     			}
3461     		} else {
3462     			/*
3463     			 * No rdatasets of the given type exist at the node.
3464     			 */
/lib/dns/qpcache.c: 3409 in add()
3403     			}
3404     			newheader->next = topheader->next;
3405     			newheader->down = topheader;
3406     			topheader->next = newheader;
3407     			qpnode->dirty = 1;
3408     			if (changed != NULL) {
>>>     CID 487884:    (DEADCODE)
>>>     Execution cannot reach this statement: "changed->dirty = true;".
3409     				changed->dirty = true;
3410     			}
3411     			mark_ancient(header);
3412     			if (sigheader != NULL) {
3413     				mark_ancient(sigheader);
3414