net: Remove SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS_SUPPORT

Now that we have converted the bridge code and the drivers to check for
bridge port(s) flags at the time we try to set them, there is no need
for a get() -> set() sequence anymore and
SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS_SUPPORT therefore becomes unused.

Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>