Remove group manager APIs from Doxygen and JavaDoc

Remove group manager APIs from Doxygen and JavaDoc

The current group manager APIs will be replaced with scene manager APIs.
So I just hide group manager APIs from API document.
Removing the codes for the replacement will be planed in the future.

Change-Id: Ie2086a974d50bb7b46ac40fa85e99ad85bfc8409
Signed-off-by: Jihun Ha <jihun.ha@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/5417
Reviewed-by: Habib Virji <habib.virji@samsung.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>