Migrate: don't check for free memory on cleanup

Cleanup just updates the config with the correct location of the
instance, or informs of its down status, but never starts it. As such
there's no point in checking for enough free memory. Actually this check
could prevent a perfectly safe cleanup operation if a node is busy.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>