Free group_lookup at finalize.

Also remove useless assignments: variable are already reset to NULL
by xbt_dict_free.
(cherry picked from commit 078e59c8562f1a12a828b8197689c1843d0b6e07)
Name the release (following frs69wq's advice).
(cherry picked from commit 1028bf295229d3a5c75a74b910ca34000d40bb79)