clean up dns_rbt

clean up dns_rbt

- create_node() in rbt.c cannot fail
- the dns_rbt_*name() functions, which are wrappers around
  dns_rbt_[add|find|delete]node(), were never used except in tests.

this change isn't really necessary since RBT is likely to go away
eventually anyway. but keeping the API as simple as possible while it
persists is a good thing, and may reduce confusion while QPDB is being
developed from RBTDB code.