Put Q_DISABLE_COPY(QMutex) back when features.thread is unavailable

Revert a part of d3638e18d396f1935804908da717ca5cb97f2eda because the
change was not needed

Task-number: QTBUG-94407
Pick-to: 6.2
Change-Id: I7cf0969d6ecc4078d38b4241ffe7d24273a6d412
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/thread/qmutex.h

@@ -305,6 +305,9 @@ public:
         Q_UNUSED(timePoint);
         return true;
     }
+
+private:
+    Q_DISABLE_COPY(QMutex)
 };
 
 class QRecursiveMutex : public QMutex {};