spec: remove the --stateful flag from the move command

Per discussion on the mailing list, we can remove the --stateful flag from
move, as it is implied by the current container state.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Merge pull request #37 from tych0/drop-stateful-move-flag

spec: remove the --stateful flag from the move command