net: remove cs_vRecvMsg

vRecvMsg is now only touched by the socket handler thread.

The accounting vars (nRecvBytes/nLastRecv/mapRecvBytesPerMsgCmd) are also
only used by the socket handler thread, with the exception of queries from
rpc/gui. These accesses are not threadsafe, but they never were. This needs to
be addressed separately.

Also, update comment describing data flow

src/net_processing.cpp

@@ -2439,7 +2439,6 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
     return true;
 }
 
-// requires LOCK(cs_vRecvMsg)
 bool ProcessMessages(CNode* pfrom, CConnman& connman, std::atomic<bool>& interruptMsgProc)
 {
     const CChainParams& chainparams = Params();