net: remove comments that mention obsolete __SLOW_DOWN_IO

The only remaining definitions of __SLOW_DOWN_IO (for alpha and ia64) do
nothing, and the only mentions in networking are in comments.  Remove these
mentions.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>