[roll] Roll fuchsia [settings] Remove older TODOs from migration.rs

We have a few older TODOs in `migration.rs` that are no longer
relevant. This removes them and adds a few comments about why we're
leaving dead code annotations in place for now.

Test: fx build
Original-Bug: 91407
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/782126
Original-Revision: 4f9ff7f19c92ee8e884ecc16deb5ad1a7e484882
GitOrigin-RevId: ae59211a8c9b739aa4b5458fd8836d7c9798083d
Change-Id: I5df777446c7c33c5625c6ce989b8d5e554c3c4d1
diff --git a/stem b/stem
index 9edd62c..f525da5 100644
--- a/stem
+++ b/stem
@@ -11,7 +11,7 @@
              githooks="integration/git-hooks"
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="1752020b73ccde3a9e85064c2ba196e5a3ac6c35"/>
+             revision="4f9ff7f19c92ee8e884ecc16deb5ad1a7e484882"/>
   </projects>
   <hooks>
     <hook name="install-environment"