drop SpamAssassin add-on

drop SpamAssassin add-on

This package has not been maintained well and is thereof outdated. At
the time of writing, we neither
(a) have a maintainer for this nor
(b) believe it is wise to run a full-featured content scanner on a
    firewall for security purposes. (We can make do with Postfix, as it
    is known for being a very robust MTA and providess less attack
    surface than something actually inspecting transferred messages.)

Thereof, this patch drops the SpamAssassin add-on. In case it is desired
in future versions of IPFire, it can be easily reverted, restoring the
functionality and behaviour before.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>