bridge: remove mode BRIDGE_MODE_SWDEV

This patch removes bridge mode swdev.
Users can use BRIDGE_FLAGS_SELF to indicate swdev offload
if needed.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>