Do not create Deleted on Workspace Shutdown

When the Workspace is shutting down the compositor is torn down
before Clients and Unmanaged are released. This means that there
is no need to create the Deleted windows.

Furthermore creating the Deleted manipulates the stacking_order
while Workspace dtor loops over this list to release all clients.
This may cause crashes.

BUG: 282933
FIXED-IN: 4.9.0
REVIEW: 104690