QHttpHeaders: remove an unnecessary std::move

The local is always moved in this situation. (I'm not sure why
this function returns a local but other functions use make_optional.)

Amends ed341fcdd18fb4bb58ea4fa9df0be6350a81578c.

Change-Id: I6719691ad13c65af083f1749be475dc8e2da4b07
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/network/access/qhttpheaders.cpp

@@ -1532,7 +1532,7 @@ std::optional<QDateTime> QHttpHeaders::dateTimeValue(QAnyStringView name) const
         return std::nullopt;
     QDateTime dt = QNetworkHeadersPrivate::fromHttpDate(*v);
     if (dt.isValid())
-        return std::move(dt);
+        return dt;
     return std::nullopt;
 }