sig: Abolish sethash and defhash everywhere

sig: Abolish sethash and defhash everywhere

The only consumer of this wrongheaded interface was rsa1, which has
been updated.  Delete it all.

There is nothing to delete in the documentation, mostly because much
of this wasn't documented when we introduced it.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>