lib/oe/package_manager.py: do not use stamps

lib/oe/package_manager.py: do not use stamps

Using stamps shortens rootfs/image creation because the package indexes
are not created again if no new package was installed in the deploy dir.
Unfortunately, there are some syncronization issues with
do_package_write_*() and the indexes are not properly created.

Will have to revisit this issue in the near future. For now, lose it.

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>