packagegroups: Remove obmc-mgr-system

The system manager (obmc-mgr-system) which provided GPIO
functionality is being deprecated and it's not in use anymore:
* GPIOs have moved out of D-Bus (see openbmc/openbmc#3332).
* Tools depending on its GPIO functions have been updated to
call local functions instead.
* The systemd service files have been updated to not depend
on system manager.

(From meta-quanta rev: 602f74d56aeabe7c30748576ad6085ed761498fe)

Change-Id: I76e683ed246bc055c54754bfc9d819f50e708eb3
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>