TextInputv3: use a serial for each resource

Each zwp_text_input_v3 object should have their own
serial number and it will not be cleared on the same object.

Pick-to: 6.7
Change-Id: I779c2ae07f1dab1ae4a7d87e4e183ce33ec804fe
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: David Edmundson <davidedmundson@kde.org>
Reviewed-by: Weng Xuetian <wengxt@gmail.com>

src/plugins/platforms/wayland/qwaylandtextinputv3.cpp

@@ -67,7 +67,6 @@ void QWaylandTextInputv3::zwp_text_input_v3_leave(struct ::wl_surface *surface)
     m_currentPreeditString.clear();
 
     m_surface = nullptr;
-    m_currentSerial = 0U;
 
     disable();
     qCDebug(qLcQpaWaylandTextInput) << Q_FUNC_INFO << "Done";