exim: revert DKIM signing for wikimedia.org

There's currently two problem with the DKIM signing on remote_smtp. The
first one is that sodium also serves as a backup MX, relaying mails to
mchenry, so it would happily sign unrelated incoming mails (e.g.  spam)
with our domain.

The second problem is that even if we switch dkim_domain to
$sender_host_address (effectively signing only with From:
@wikimedia.org), it will still happily accept spoofed mails with From
@wikimedia.org To: @wikimedia.org and sign them, which isn't really the
intended behavior.

So we really need to not sign relayed traffic at all and this can't
happen with some refactoring & splitting transports. Revert this for
now, as it's really wrong to sign potentially spoofed mails in the
meantime.

(this has been reverted manually soon after signing was merged, so this
wasn't live for long)

Change-Id: I3f04c3e7ac05881532f83cd34628a69f9c925b54