Remove dead nVersion code

This is very old code from the early days of Bitcoin, our mainnets have
never used peer protocol version.

Originally 150ab1d34c

src/main.cpp

@@ -6915,8 +6915,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         int nVersion;           // use temporary for version, don't set version number until validated as connected
         const int minVersion = ishush3 ? MIN_HUSH_PEER_PROTO_VERSION : MIN_PEER_PROTO_VERSION;
         vRecv >> nVersion >> pfrom->nServices >> nTime >> addrMe;
-        if (nVersion == 10300)
-            nVersion = 300;
+
         if (nVersion < minVersion)
         {
             // disconnect from peers older than this proto version