throttle: Remove throttle_group_lock/unlock() The group throttling code was always meant to handle its locking internally. However, bdrv_swap() was touching the ThrottleGroup structure directly and therefore needed an API for that. Now that bdrv_swap() no longer exists there's no need for the throttle_group_lock() API anymore. Signed-off-by: Alberto Garcia <berto@igalia.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>