Remove unused instance variable from occurrence_map

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/176024



Merged-by: Michael Becker <mbecker@gitlab.com>
Approved-by: Michael Becker <mbecker@gitlab.com>
Reviewed-by: Michael Becker <mbecker@gitlab.com>
Co-authored-by: Zamir Martins Filho <zfilho@gitlab.com>