Update a few comments in ct-server.cc.

It's true that the SignMerkleTree function would call LogLookup<>::Update,
but that's not the real reason we need to call it at startup. LogLookup
loaded whatever was in the database in its constructor already, the real
problem is when there's nothing at all in the database, we'd serve an invalid
(unsigned) empty STH.
Maybe we should just remove this call now? It's gotta go at some point and we're going to have all the log bits spread all over the the kitchen table for a bit anyway so it's unlikely to be otherwise working...