csip: avoid create two group with same id

Summary:
- When first device connected for a new csis group, csip stack is
possible to create two group with same id.

Reason:
- In OnCsisSirkValueUpdate(), if the group not found
(means group_id is unknown), it will call DeviceGroups::AddDevice(),
and OnGroupAddedCb() be raised, a new csis group will be made in
AssignCsisGroup().
- But in OnCsisSirkValueUpdate(), it still creating a new csis group
without checking if the group is already created, this causes another
new group with same id will be added to csis_groups_.

Solution:
- We should remove it and ensure create new group only in
AssignCsisGroup().

Change-Id: Iaff4a54a0ea44d67f256052e7be9ffa47f11407d