Revert "Move keep-above clients to the Normal layer when showing desktop"

This reverts commit 1e2a0028c3151cb38f252baa78a5f22b8cd7c7e6.

Unfortunately, we can't move clients from the above layer to the normal
layer because some of those clients have to be visible when showing
desktop, one such client for example is krunner.

CCBUG: 406101