datapath: Don't track IP TOS value two different ways.
Originally, the datapath didn't care about IP TOS at all. Then, to support
NetFlow, we made it keep track of the last-seen IP TOS value on a per-flow
basis. Then, to support OpenFlow 1.0, we added a nw_tos field to
odp_flow_key. We don't need both methods, so this commit drops the
NetFlow-specific tracking.
This introduces a small kernel ABI break: upgrading the kernel module
without upgrading the OVS userspace will mean that NetFlow records will
all show an IP TOS value of 0. I don't consider that to be a serious
problem.