ospack: Drop chaiwala-bootflags

The current rollback mechanism has been unsupported for quite some time,
let's temporarily drop it from the images until a new, better
ostree-based mechanism emerges.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>
Reviewed-by: Héctor Orón Martínez <hector.oron@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D7272