Remove dns.rdata{type,class}.to_enum.

Remove dns.rdata{type,class}.to_enum.

These methods (which convert a str/int into an enum/int) shouldn't be
commonly used by external code, so don't need to exist at the module
level.  The make() method on the enum class (renamed from to_enum()) can
still be used, and the internal callers have been updated to use it.