wayland: Don't call updateDecoration() for popups
After lowering the message severity for the KWIN_CORE logging category, it has been found that the Workspace calls the updateDecoration() method for every mapped popup surface.
Since the XdgPopupClient doesn't implement that method, a warning message is printed every time a popup is mapped.
Given that XdgToplevelClient updates the decoration when needed, we can remove the call to the updateDecoration() method.