Otherwise we may skip saving a target image to the db when updating from legacy supervisors,
which in turn prevents from deleting the legacy image entry (with imageId = 1), leaving the
supervisor in a state where it can't report its current state to the API.
While we're at it, we also remove an unused variable in _getStatus.
Change-type: patch
Signed-off-by: Pablo Carranza Velez <pablo@balena.io>