kernel: fix bit clearing logic in _k_thread_group_leave

Fix init_group bit clearing in _k_thread_group_leave()

Fix _k_object_uninit calling order. Though the order won't
make much difference in this case it is always good to destroy
or uninitialize in the reverse order of the object creation or
initialization.

Signed-off-by: Ramakrishna Pallala <ramakrishna.pallala@intel.com>