screen: Remove stray assert

When the number of (static) workspaces decreases, we relocate windows
from extra workspaces before removing them. As removing a non-empty
workspace is not allowed, we assert that it doesn't contain any windows
before removing it.

However that assert is

 - pointless, because meta_workspace_remove() already asserts that
   the workspace is empty

 - wrong, because even empty workspaces contain windows that are set
   to show on all workspaces

Simply drop the assert to avoid a crash when trying to remove a workspace
while on-all-workspaces windows are present.

https://bugzilla.gnome.org/show_bug.cgi?id=784223