remove dead rpc code

Checking for fHelp, or the size of the args, is dead code because:
* fHelp is always false (src/qt/test/rpcnestedtests.cpp)
* It is already implicitly called by RPCHelpMan::Check
  (src/rpc/mining.cpp, src/rpc/misc.cpp, src/rpc/net.cpp)

src/qt/test/rpcnestedtests.cpp

@@ -16,9 +16,6 @@
 
 static UniValue rpcNestedTest_rpc(const JSONRPCRequest& request)
 {
-    if (request.fHelp) {
-        return "help message";
-    }
     return request.params.write(0, 0);
 }