container: Remove Ghost state

container.Register() checks both IsRunning() and IsGhost(), but at
this point IsGhost() is always true if IsRunning() is true. For a
newly created container both are false, and for a restored-from-disk
container Daemon.load() sets Ghost to true if IsRunning is true. So we
just drop the IsGhost check.

This was the last call to IsGhost, so we remove It and all other
traces of the ghost state.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #5335 from alexlarsson/remove-ghost

container: Remove Ghost state