datapath: Don't track IP TOS value two different ways.

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.