Remove the ANCIENT slabheader attribute and statistics counter

Remove the ANCIENT slabheader attribute and statistics counter

Cache headers are now unlinked from their node as soon as they expire,
so a slabheader is never left in the "ancient, awaiting cleanup" state
that DNS_SLABHEADERATTR_ANCIENT tracked.  Drop the attribute, rename
mark_ancient() to header_delete() to reflect that it now removes the
header rather than flagging it (keying idempotency on list membership),
and remove the ancient RRset statistics counter that recorded the state,
which is now always zero; the rdataset statistics array shrinks to
match.

The rdataset-level 'ancient' flag and 'rndc dumpdb -expired' are
unaffected: expiry is derived from the entry's TTL when the rdataset is
bound, not from the slabheader attribute.