Remove dns_rdataslab_tordataset() and its related dns_rdatasetmethods_t callbacks

lib/dns/rdataslab.c contains 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 lib/dns/rbtdb.c. Since 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 dns_rdataslab_fromrdataset() which lib/dns/rbtdb.c uses for inserting data into memory. But the slabs created using that method are never "exported" using dns_rdataslab_tordataset() - bind_rdataset() is used for that purpose.