tabbox: Remove Xcb::sync()

TabBox::hide() used to contain the following code

    QApplication::syncX();
    XEvent otherEvent;
    while (XCheckTypedEvent(display(), EnterNotify, &otherEvent))
        ;

The purpose of XSync() is to flush any pending requests and wait until
the X server processes them. After that, we can go through event queue
to find relevant events.

Why did tabbox go through the event queue to find the EnterNotify
events? It's unclear. So it was removed in
b4c957b617. However, the XSync() call was
left out. It doesn't make sense on its own. So remove it too.

src/tabbox/tabbox.cpp

@@ -725,9 +725,6 @@ void TabBox::hide(bool abort)
         qCDebug(KWIN_TABBOX) << "Tab box was not properly closed by an effect";
     }
     m_tabBox->hide(abort);
-    if (kwinApp()->x11Connection()) {
-        Xcb::sync();
-    }
 }
 
 void TabBox::reconfigure()