netfilter: bridge: remove BRNF_STATE_BRIDGED flag

Its not needed anymore since 2bf540b7
([NETFILTER]: bridge-netfilter: remove deferred hooks).
Before this it was possible to have physoutdev set for locally generated
packets -- this isn't the case anymore:

BRNF_STATE_BRIDGED flag is set when we assign nf_bridge->physoutdev,
so physoutdev != NULL means BRNF_STATE_BRIDGED is set.
If physoutdev is NULL, then we are looking at locally-delivered and
routed packet.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>