chore(tvix/[ca]store): Drop sled support completely in favor of redb

Over the past couple of months we've been using redb instead of sled as
the default filesystem-based database in PS and DS. I am confident that
we can get rid of sled completely now, and just keep redb.

Change-Id: I11fa1e4453e280253855f8eade990b37eb6965ae
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12567
Reviewed-by: yuka <yuka@yuka.dev>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Autosubmit: Ilan Joselevich <personal@ilanjoselevich.com>

tvix/castore/Cargo.toml

@@ -19,7 +19,6 @@ object_store = { workspace = true, features = ["http"] }
 parking_lot = { workspace = true }
 pin-project-lite = { workspace = true }
 prost = { workspace = true }
-sled = { workspace = true }
 thiserror = { workspace = true }
 tokio-stream = { workspace = true, features = ["fs", "net"] }
 tokio-util = { workspace = true, features = ["io", "io-util", "codec"] }