6pack: drop sixpack::mtu

6pack: drop sixpack::mtu

It holds a constant (AX25_MTU + 73), so use that constant in place of
the single use directly.

And remove the stale comment.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Andreas Koensgen <ajk@comnets.uni-bremen.de>
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: linux-hams@vger.kernel.org
Cc: netdev@vger.kernel.org
Reviewed-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://lore.kernel.org/r/20240808103549.429349-9-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>