remove the close action

@dinhviethoa, this removes the context menu's 'Close' item. It's fairly broken:

- sync-fs resources that you close, show up again the next time you start spark (#1037)
- closing a project doesn't close the associated open editors
- if you do close a sync-fs project, there's no way for the user to later re-open it

I'd like to remove this functionality until it's working better (and push re-enabling it off to a later milestone).