Remove NumBlocksOfPeers

Generally useless information. Only updates on connect time, not after that. Peers can easily lie and the median filter is not effective in preventing that.

In the past it was used for progress display in the GUI but `CheckPoints::guessVerificationProgress` provides a better way that is now used. It was too easy to mislead it. Peers do lie about it in practice, see issue #4065.

From the RPC, `getpeerinfo` gives the peer raw values of the initial height, which are more useful. I've also suggested in #4133 to show the peer information in a GUI table.

(for the discussion on IRC that sparked this, see #4127)