Drop JSONRPCRequest constructors after #21366

This just makes an additional simplification after #21366 replaced
util::Ref with std::any. It was originally suggested
https://github.com/bitcoin/bitcoin/pull/21366#issuecomment-792044351 but
delayed for a followup. It would have prevented usage bug
https://github.com/bitcoin/bitcoin/pull/21572.

src/bitcoind.cpp

@@ -224,7 +224,7 @@ static bool AppInit(int argc, char* argv[])
             // If locking the data directory failed, exit immediately
             return false;
         }
-        fRet = AppInitInterfaces(node) && AppInitMain(context, node);
+        fRet = AppInitInterfaces(node) && AppInitMain(node);
     }
     catch (const std::exception& e) {
         PrintExceptionContinue(&e, "AppInit()");