remove find_deepest_zonecut() from qpcache

remove find_deepest_zonecut() from qpcache

because the cache no longer stores delegation (parent-side) NS rrsets,
and authoritative (child-side) NS rrsets don't affect recursion,
it no longer makes sense for qpcache_find() to look for NS rrsets
and return DNS_R_DELEGATION. that code has been removed.

the cache still does search for covering DNAME records. the
check_zonecut() function has been renamed to check_dname() for clarity.

related changes:
- one test case has been removed from the mirror system test, because it
  tested the behavior of a cached delegation.
- query_checkrrl() and rpz_rrset_find() have been updated so they no
  longer expect cache responses to have DNS_R_DELEGATION response codes.