wasm: remove message handler lock

We have now upgraded the minimum emsdk to one where Emscripten properly
synchronizes stdout/stderr output.

Pick-to: 6.4
Change-Id: I537897b31916a52303a3fdb1d255f03b4aeaffef
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/corelib/global/qlogging.cpp

@@ -1789,13 +1789,6 @@ static void stderr_message_handler(QtMsgType type, const QMessageLogContext &con
     if (formattedMessage.isNull())
         return;
 
-#ifdef Q_OS_WASM
-    // Prevent thread cross-talk, which causes Emscripten to log
-    // non-valid UTF-8. FIXME: remove once we upgrade to emsdk > 2.0.30
-    Q_CONSTINIT static QBasicMutex m;
-    auto locker = qt_unique_lock(m);
-#endif
-
     fprintf(stderr, "%s\n", formattedMessage.toLocal8Bit().constData());
     fflush(stderr);
 }