Remove code_suggestions_usage_events_in_pg feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175836



Merged-by: Rémy Coutable <remy@rymai.me>
Approved-by: Emerald-Jayde Henao <ejhenao@gitlab.com>
Approved-by: Rémy Coutable <remy@rymai.me>
Reviewed-by: Rémy Coutable <remy@rymai.me>
Co-authored-by: Pavel Shutsin <pshutsin@gitlab.com>