Documentation: networking: dsa: Remove poll_link description

This function has been removed in 4baee937 ("net: dsa: remove DSA
link polling") in favor of using the PHYLIB polling mechanism.

Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>