[ietf-dkim] Issue 1543: remove [FWS]

[Stephen Farrell]

Issue descritption: https://rt.psg.com/Ticket/Display.html?id=1543

ssp-04 still includes the FWS, I didn't see a clear consensus on the
list either way (or else I missed some messages;-)

If we really need a strawpoll/consensus check on this please say so
and I'll start one.

Otherwise, I'll assume its ok to stick with FWS and ask Eliot to
close this one on July 11.

S.