Obsolete the "tkey-domain" statement

Obsolete the "tkey-domain" statement

The "tkey-domain" statement has effectively been a no-op since commit
bd4576b3cef88bcb78ae0dd7619019be4fdfb2ea, which removed the only bit of
code using it: the logic implementing TKEY Mode 2 (Diffie-Hellman).

A subsequent cleanup commit, 885c132f4ae6d895ee973648e15d9e61d01555a8,
also missed the opportunity to remove the "tkey-domain" statement
altogether.

Mark the "tkey-domain" statement as obsolete and remove all code and
documentation related to it.