Both notifyNewTopic and notifyPostChange trigger mention notif

When posting a new topic with a reply, it would trigger two identical
mention notifications. We don't need to trigger it in new topic because
it will be always covered by the post change

Change-Id: I3d58f6391d953401a65a89e4b10530f36b83c563
(cherry picked from commit 1c0ce37b74195e761ab4d281c41c06f082d1f01b)