Simplify disk encryption question

Simplify disk encryption question

The yes/no question was mistaken as actual passphrase prompt and/or details
in parentheses were taken as option list (despite the lack of commas).

Unmention the only disk encryption mechanism we support and simply ask
whether to protect the root disk with a passphrase or not (still yes/no).

Prodded by solene, feedback from many
Wording from naddy, similar wording from sthen
OK naddy sthen deraadt afresh1