package_manager: rootfs: remove install_order

package_manager: rootfs: remove install_order

The install_order instance variable was only ever set to
Manifest.INSTALL_ORDER, modified nowhere and used as is. Instead of
using an instance variable use Manifest.INSTALL_ORDER directly to
simplify the code for those reading it.

No functional changes.

Signed-off-by: Rouven Czerwinski <rouven.czerwinski@linaro.org>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>