In commit 2723e0e2/PR #26162, we created a new version of the
`anonymous_id_for_user` method that would make it safe to rotate the
`SECRET_KEY` setting (without changing existing anonymous IDs). This
included a toggle to allow fast reversion to the old code if needed.
This commit removes the old code and the toggle as we are satisfied with
the correctness and performance of the new code.
ref: ARCHBOM-1645
ac7a9f405b