Cleanup: Remove useless Server.IsRunning and Server.SetRunning

The only side-effect is that it makes an existing race condition
slightly more likely to happen. The difference in impact is negligible
whereas the impact on code cleanup is significant. The actual fix
for the race condition is to implement a better auto-restart system.
There is a patch under a review which does just that.

Signed-off-by: Solomon Hykes <solomon@docker.com>