Mark CFFI for encrypting the database as deprecated

[link2xt]

Experimental database encryption was removed on iOS and Android:
deltachat/deltachat-ios#2010
deltachat/deltachat-ios#2011

This database encryption is also an incomplete feature because blobs are not encrypted.

Delta Touch is using CFFI API with a manually entered password because Ubuntu Touch does not offer filesystem or disk encryption, but we don't want new users of these APIs, such as bot developers.

JSON-RPC for this does not exist and PR adding it is closed at #6141