net: dsa: Remove documentation for port_fdb_prepare

This callback was removed some time ago, also remove the documentation.

Fixes: 1b6dd556 ("net: dsa: Remove prepare phase for FDB")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>