overview: Remove desktop fade logic

overview: Remove desktop fade logic

It existed to fade out/in `nautilus-desktop` for the overview, but it only
ever worked for X11 sessions (`Meta.WindowType.DESKTOP`) and
`nautilus-desktop` no longer exists anyway.

While I had suggested extending it in the past (!1395), that work was
never finished and since then the DING extension has implemented its
own visibility toggling. There seems little value in keeping the old
fade logic around in gnome-shell. Removing it actually fixes a bug with
DING (https://launchpad.net/bugs/1965072).

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2244>