Remove dns_rdataslab_tordataset() and its related dns_rdatasetmethods_t callbacks
dns_rdataslab_tordataset(), a function which allows an RDATA slab to be converted to a
dns_rdataset_t backed by a set of methods which are almost exact duplicates of
rdataset_*() routines found in
dns_rdataslab_tordataset() is not used anywhere in the tree and, as of BIND 9.13, libdns is no longer considered a public library, that function and its related set of
dns_rdatasetmethods_t callbacks can be removed.
As an aside, note that the story is entirely different when it comes to
lib/dns/rbtdb.c uses for inserting data into memory. But the slabs created using that method are never "exported" using
bind_rdataset() is used for that purpose.