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-inspur rev: dd16cd4cce9956f5a7e1115bc268bcac4a3647f5)

Change-Id: Id87ff99546c55947be3db006583a99edf2b171e7
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>