Remove docs for removed can_encrypt_volumes setting

Change I730b4ffc26f391065943cb0d135ceac113cb6754 removed
support for encrypted volumes, but didn't remove the docs.

Change-Id: I29854e9d0abcccbfe3949797908016780b47ff6c
Closes-bug: #1250711
Merge "Remove docs for removed can_encrypt_volumes setting"