rpc: Remove chain-specific RequireRPCPassword

I've never liked the chain-specific exception to having to set a
password. It gives issues with #6388 which makes it valid to
set no password in every case (as it enables random cookie authentication).

This pull removes the flag, so that all chains are regarded the same.

It also removes the username==password test, which doesn't provide any
substantial extra security.

src/rpcserver.cpp

@@ -598,8 +598,7 @@ void StartRPCThreads()
     LogPrint("rpc", "Allowing RPC connections from: %s\n", strAllowed);
 
     strRPCUserColonPass = mapArgs["-rpcuser"] + ":" + mapArgs["-rpcpassword"];
-    if (((mapArgs["-rpcpassword"] == "") ||
-         (mapArgs["-rpcuser"] == mapArgs["-rpcpassword"])) && Params().RequireRPCPassword())
+    if (mapArgs["-rpcpassword"] == "")
     {
         unsigned char rand_pwd[32];
         GetRandBytes(rand_pwd, 32);