Remove AutofillTable::RemoveOrphanAutofillTableRows()

Local profiles were migrated to a new table layout, so the old table is
no longer used. RemoveOrphanAutofillTableRows() thus no longer has a
purpose, since it only acted as a fix for some bug in the old table
layout (see crbug.com/836737).

The pref kAutofillOrphanRowsRemoved becomes obsolete due to this change.
As outlined in [1], it should not be removed directly, but rather
cleared first, before it is removed in a year. Conveniently, Autofill
already has a helper function MigrateDeprecatedAutofillPrefs() for that,
so no changes in browser_prefs.(cc|m) are necessary.

[1] https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/prefs/README.md

Bug: 1443393
Change-Id: I70019a3db79c14d4b27e92cd37c5eaf93793e0fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4526396


Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1148381}