Please cherry-pick 9b7c68b3911aef84afa4cbfc31bce20f10570d51 ("netfilter: ctnetlink: Support offloaded conntrack entry deletion")

Please cherry-pick 9b7c68b3911aef84afa4cbfc31bce20f10570d51 ("netfilter: ctnetlink: Support offloaded conntrack entry deletion") to all supported stable trees except for 4.14. The lack of it makes the flowtables feature much more difficult (if not impossible) to use in environments where connection tracking entries must be removed to terminate flows. The diffstat is -8,+0 and the commit only removes code that was not necessary to begin with.