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.