QtDBus: Remove assertion inside qDBusAddTimeout()

The assertion is triggered when a timer cannot be started. This
is normal occurrence when an application exits before all pending
D-Bus calls are processed. And there is a code handling such failure
in the same function. Qt also prints a warning message in this
case:

    QEventDispatcherGlib::registerTimer: invalid arguments

Task-number: QTBUG-58732
Change-Id: I37859443fa90ae1bf7da1572ac9b02a54c8e1b99
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 90e0c6693b3bd9c5272bd8e6255ca9aefd183912)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/dbus/qdbusintegrator.cpp

@@ -125,7 +125,6 @@ static dbus_bool_t qDBusAddTimeout(DBusTimeout *timeout, void *data)
     Q_ASSERT(d->timeouts.key(timeout, 0) == 0);
 
     int timerId = d->startTimer(std::chrono::milliseconds{q_dbus_timeout_get_interval(timeout)});
-    Q_ASSERT_X(timerId, "QDBusConnection", "Failed to start a timer");
     if (!timerId)
         return false;