Treat RTM_DELNEIGH as a transition to NUD_NONE

RTM_DELNEIGHs can happen naturally during garbage collection of STALE
entries, as well as when an interface is going down.  All other
transitions of interest at this time pass through FAILED.

Bug: 18581716
Bug: 21531726
Change-Id: Ib2a731dd6ad6ece478df6950d4cc7752588328a4