Code health: Clean up stale base::Feature "ReadLaterBackendMigration"

As this was the last feature in reading_list::switches and the reading
list is now supported on all platforms, this CL also removes the entire
components/reading_list/features folder (and all references to it).

Bug: 356236922
Change-Id: I95a36ef55cedd429c9c7a0e97f05f805946480a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6494258
Reviewed-by: Ankush Singh <ankushkush@google.com>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Caroline Rising <corising@chromium.org>
Reviewed-by: Darryl James <dljames@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Adam Arcaro <adamta@google.com>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1454460}