Increase signal to noise in appveyor build output by reducing the MSVC warning count from 12 to 4 (12 is assuming the changes in #14086 are also implemented).

This makes it easier to spot errors or more important warnings in the verbose appveyor output.

See https://github.com/bitcoin/bitcoin/pull/14086#issuecomment-416610313 plus discussion for context.

src/wallet/rpcwallet.cpp

@@ -288,7 +288,6 @@ static UniValue setlabel(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid Bitcoin address");
     }
 
-    std::string old_label = pwallet->mapAddressBook[dest].name;
     std::string label = LabelFromValue(request.params[1]);
 
     if (IsMine(*pwallet, dest)) {