dpif-netdev: Remove max_mtu tracking.

dpif-netdev: Remove max_mtu tracking.

Normally all the ports have the same mtu anyhow, so there is little
advantage in keeping track of the maximum mtu on a per-bridge basis.  In
upcoming commits, tracking mtu will require more locking and present
even less advantage (because the packet buffer will become per-thread, so
that reallocating once per thread becomes essentially a null cost).

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>