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, which are more
useful.

src/qt/clientmodel.h

@@ -60,8 +60,6 @@ public:
     bool inInitialBlockDownload() const;
     //! Return true if core is importing blocks
     enum BlockSource getBlockSource() const;
-    //! Return conservative estimate of total number of blocks, or 0 if unknown
-    int getNumBlocksOfPeers() const;
     //! Return warnings to be displayed in status bar
     QString getStatusBarWarnings() const;
 
@@ -75,7 +73,6 @@ private:
     OptionsModel *optionsModel;
 
     int cachedNumBlocks;
-    int cachedNumBlocksOfPeers;
     bool cachedReindexing;
     bool cachedImporting;
 
@@ -88,7 +85,7 @@ private:
 
 signals:
     void numConnectionsChanged(int count);
-    void numBlocksChanged(int count, int countOfPeers);
+    void numBlocksChanged(int count);
     void alertsChanged(const QString &warnings);
     void bytesChanged(quint64 totalBytesIn, quint64 totalBytesOut);