layer/iterate: remove counter-productive validation

... functionality from iterator: don't fail immediately if actual number
of labels in owner name exceeds number in label field of RRSIG rrset
5 jobs for iterate-rrsiglabels in 2 minutes and 8 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
canceled build:linux:amd64 #73016
linux docker amd64

passed build:linux:amd64 #30944
docker linux amd64

00:00:30

 
  Test
passed deckard:linux:amd64 #30946
docker linux amd64

00:00:35

passed respdiff:linux:amd64 #30947
docker linux amd64

00:02:08

passed test:linux:amd64 #30945
docker linux amd64

00:00:22