Simplify Window::updateLayer()

The stack layer of a transient doesn't depend on its transient parent.

It is not clear why this code exists, it dates back to a commit 20 years
ago, which is massive and difficult to analyze. But, with that being said,
Window::belongsToLayer() doesn't place transients in parent's stack
layer, stack constraints are used instead.

src/window.cpp

@@ -553,9 +553,6 @@ void Window::updateLayer()
     }
     StackingUpdatesBlocker blocker(workspace());
     m_layer = UnknownLayer; // invalidate, will be updated when doing restacking
-    for (auto it = transients().constBegin(), end = transients().constEnd(); it != end; ++it) {
-        (*it)->updateLayer();
-    }
 }
 
 Layer Window::belongsToLayer() const