net: dsa: remove switch routing table setup code

The dsa_switch structure has no routing table specific data to setup,
so the switch fabric can directly walk its ports and initialize its
routing table from them.

This allows us to remove the dsa_switch_setup_routing_table function.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>