[Autofill] Remove AutofillHighlightOnlyChangedValuesInPreviewMode

The feature is not needed anymore. It has been disabled by default
never experimented with since 2021.

Bug: 326733459, 1248585, 1247978
Change-Id: Ibca8809ce0208a101baee95b7f1d109541983636
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5317884


Reviewed-by: Matthias Körber <koerber@google.com>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1267486}