desktop-shell: Remove multiple workspace support

It's not the most code ever, but it does make desktop-shell somewhat
more complicated for questionable (i.e. no) end-user benefit.

When desktop-shell is back in more healthy shape it could potentially be
reintroduced, but for now it's just making it more difficult to reason
about desktop-shell and fix it.

Signed-off-by: Daniel Stone <daniels@collabora.com>