refactor: Remove unneeded deprecated attr

[Hocuri]

Was marked as deprecated as of #7489 (comment), but since it's still in use (and not discouraged to use), I just stumbled over it while browsing the code.

[iequidoo]

It's used by dc_context_open(), but it has a doc comment:

 * NB: Nonempty passphrase (db encryption) is deprecated 2025-11:
 * - Db encryption does nothing with blobs, so fs/disk encryption is recommended.
 * - Isolation from other apps is needed anyway.

Also it's used internally, but this can just be refactored, nonempty password support isn't needed for internal usage. I.e. it's actually deprecated as a public API.