Remove autofill assistance toggle from PCO settings page

The Compose feature will not use PCO - the pref and corresponding settings UI are no longer relevant.
This effectively reverts https://crrev.com/c/4932521

Bug: b:315378546
Change-Id: I5a8f2e53a2e203998c9398175e60606d413f17b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5170765
Commit-Queue: Anthony Cui <[email protected]>
Reviewed-by: John Lee <[email protected]>
Reviewed-by: Colin Blundell <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1244152}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index b4c21f8..e406d936 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -945,6 +945,7 @@
 #if BUILDFLAG(ENABLE_COMPOSE)
 constexpr char kPrefHasAcceptedComposeConsent[] =
     "compose_has_accepted_consent";
+constexpr char kAutofillAssistanceEnabled[] = "autofill_assistance.enabled";
 #endif
 
 // Deprecated 01/2024.
@@ -1331,6 +1332,7 @@
 // Deprecated 01/2024.
 #if BUILDFLAG(ENABLE_COMPOSE)
   registry->RegisterBooleanPref(kPrefHasAcceptedComposeConsent, false);
+  registry->RegisterBooleanPref(kAutofillAssistanceEnabled, false);
 #endif
 
   // Deprecated 01/2024.
@@ -2553,6 +2555,7 @@
   // Added 01/2024.
 #if BUILDFLAG(ENABLE_COMPOSE)
   profile_prefs->ClearPref(kPrefHasAcceptedComposeConsent);
+  profile_prefs->ClearPref(kAutofillAssistanceEnabled);
 #endif
 
   // Added 01/2024.