Ignore GETBLOCKTXN requests for unknown blocks

Don't disconnect peers, or else we leak information that could be
used for fingerprinting.

src/main.cpp

@@ -5343,7 +5343,6 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
 
         BlockMap::iterator it = mapBlockIndex.find(req.blockhash);
         if (it == mapBlockIndex.end() || !(it->second->nStatus & BLOCK_HAVE_DATA)) {
-            Misbehaving(pfrom->GetId(), 100);
             LogPrintf("Peer %d sent us a getblocktxn for a block we don't have", pfrom->id);
             return true;
         }