Remove TKEY Diffie-Hellman authentication

Although draft-eastlake-dnsop-rfc2930bis-tkey hasn't been finished, I would suggest that we go with the recommendation in the draft:

4.2 Diffie-Hellman Exchanged Keying (Deprecated)

   The use of this mode (#2) is NOT RECOMMENDED for the following two
   reasons but the specification is still included in Appendix A in case
   an implementation is needed for compatibility with old TKEY
   implementations. See Section 4.6 on ECDH Exchanged Keying.

      The mixing function used does not meet current cryptographic
      standards because it uses MD5 [RFC6151].

      RSA keys must be excessively long to achieve levels of security
      required by current standards.