Revert "Store a z-index id per-layer (#4128)"

This reverts commit e4065c45cb, reversing
changes made to 6c2236ae1b.

crates/workspace/src/workspace.rs

@@ -4334,7 +4334,7 @@ impl Element for DisconnectedOverlay {
     }
 
     fn paint(&mut self, bounds: Bounds<Pixels>, overlay: &mut Self::State, cx: &mut WindowContext) {
-        cx.with_z_index(u16::MAX, |cx| {
+        cx.with_z_index(u8::MAX, |cx| {
             cx.add_opaque_layer(bounds);
             overlay.paint(cx);
         })