Remove linked_work_items feature flag

See merge request !150148



Merged-by: Miranda Fluharty <mfluharty@gitlab.com>
Approved-by: Arpit Gogia <12347103-arpitgogia@users.noreply.gitlab.com>
Approved-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Approved-by: Diana Zubova <dzubova@gitlab.com>
Approved-by: Miranda Fluharty <mfluharty@gitlab.com>
Reviewed-by: Diana Zubova <dzubova@gitlab.com>
Reviewed-by: Arpit Gogia <12347103-arpitgogia@users.noreply.gitlab.com>
Co-authored-by: Eugenia Grieff <egrieff@gitlab.com>