[CBD] Clean unused kPreferencesMigratedToBasic pref

Bug: 331342247
Change-Id: I4421328f105da8965930541404fc17de0e47a71e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402871


Auto-Submit: Filipa Senra <fsenra@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280121}