fix(styles): remove unused var(--newtab-text-tertiary-color) (#4193)
fix(styles): remove unused var(--newtab-text-tertiary-color) (#4193)

Followup Bug 1463381
mozilla/activity-streams,mozilla/activity-stream,sarracini/activity-stream,rlr/activity-streams,AdamHillier/activity-stream,rlr/activity-streams,mozilla/activity-stream,AdamHillier/activity-stream,rlr/activity-streams,rlr/activity-streams,Mardak/activity-stream,mozilla/activity-streams,sarracini/activity-stream,Mardak/activity-stream,AdamHillier/activity-stream,sarracini/activity-stream,Mardak/activity-stream,AdamHillier/activity-stream,Mardak/activity-stream,sarracini/activity-stream,mozilla/activity-stream,mozilla/activity-stream