These were not thread safe, and could not be: to be useful, a thread looking up a time
conversion could cache the result, but it would be using the global (shared) copy
of the map (because lookup is read-only, and so no write-copy is required). But
inserting into the lookup table wasn't lock protected (and shouldn't be because otherwise
that defeats the point of RCU).
So just drop it.
52f1b88749