Remove network state wipe from UnloadBlockIndex.

UnloadBlockIndex is only used during init if we end up reindexing
to clear our block state so that we can start over. However, at
that time no connections have been brought up as CConnman hasn't
been started yet, so all of the network processing state logic is
empty when its called.

Additionally, the initialization of the recentRejects set is moved
to InitPeerLogic.