Older chatmail addresses may have "move to deltachat folder" enabled and can not disable it anymore

[hpk42]

We have recently moved to hide the "move to deltachat folder" option on chatmail addresses in all UIs.

Moreover, the expire script on chatmail relays does not look into sub folders, so messages in the folder never get removed.

As we mid-term want to get rid of the option, i think it's best to consider core clearing the option with the next release. This way, no UI and no chatmail user need to do anything to heal the situation.

[r10s]

ftr, this will be fixed by #7624, cmp #7622