Stop taking time-based APT snapshots of Stretch

Once we don’t use them anymore, we should drop these snapshots and stop updating them, to free tons of disk space on apt.lizard.

Doc: https://tails.boum.org/contribute/APT_repository/time-based_snapshots/#stop-tracking-a-distribution

Related issues

• Blocks #13242
• Blocked by #16868 (closed)

Original created by @intrigeri on 17165 (Redmine)