What's this persistent Camera-app notification in Android 6.0?
Android M (nexus 5, 6.0.1, security patch 1 juin 2016):

long press the notification, click the (i) button. turn on "block all", then turn off "block all".

This will dismiss the persistent notification.

thanks @Firelord.

ps: I cannot comment or vote on this SE, so I posted an answer.