[Card Benefits] Deprecate old shown form event metrics

We are migrating the card benefit form event enums to a new,
benefit-specific enum class and logging to new histograms. Previous CLs
have added the new form event enums and logging, and we are currently
double-logging metrics to both the old and new histograms.
   Old: Autofill.FormEvents.CreditCard.WithBenefits.*
   New: Autofill.FormEvents.CreditCard.Benefits.*

This CL removes the logging for the old "shown" form event enums.

Bug: 417228483
Change-Id: Ice9ef0f04595d841a69a63f6dfc8c7916de3a193
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6974414
Reviewed-by: Vinny Persky <vinnypersky@google.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Fernando Ramirez <ferny@google.com>
Cr-Commit-Position: refs/heads/main@{#1520385}