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.