The "tkey-domain" statement has effectively been a no-op since commit
bd4576b3ce, which removed the only bit of
code using it: the logic implementing TKEY Mode 2 (Diffie-Hellman).
A subsequent cleanup commit, 885c132f4a,
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.
805f1c0f65