workspace: Always remove removed windows

Since mutter commit 527c53a2a0582eba, MetaWorkspace::window-removed
is emitted *before* MetaWindow:workspace is updated, so the test
whether the removed window should still be on the workspace in
question will always return true.
Assume the test is no longer necessary nowadays to fix this very
obvious regression.

https://bugzilla.gnome.org/show_bug.cgi?id=735608