Delete kUnifiedPasswordManagerLocalPasswordsMigrationWarning flag

kUnifiedPasswordManagerLocalPasswordsMigrationWarning was *disabled*
by default in M132 (https://crrev.com/c/5928711, http://cl/686529557).
It can be cleaned up now.

Some other leftovers are removed:
- signin::AccessPoint::kPasswordMigrationWarningAndroid
- PasswordManager.PasswordMigrationWarning.Export.* histograms.

OBSOLETE_HISTOGRAMS=The migration warning UI was removed along with its histograms.

Fixed: 413696987
Change-Id: If795de648a5694f1e6ff96d8712be7e9bfb451e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6494366
Reviewed-by: Ryan Sultanem <[email protected]>
Auto-Submit: Victor Vianna <[email protected]>
Reviewed-by: Ivana Žužić <[email protected]>
Reviewed-by: David Roger <[email protected]>
Commit-Queue: Victor Vianna <[email protected]>
Reviewed-by: Mohamed Amir Yosef <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1453231}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index 9a00400..7454e42 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -1096,6 +1096,10 @@
 // Deprecated 04/2025.
 constexpr char kObsoleteUserAcknowledgedLocalPasswordsMigrationWarning[] =
     "user_acknowledged_local_passwords_migration_warning";
+
+// Deprecated 04/2025.
+constexpr char kObsoleteLocalPasswordMigrationWarningPrefsVersion[] =
+    "local_passwords_migration_warning_reset_count";
 #endif
 
 // Register local state used only for migration (clearing or moving to a new
@@ -1541,6 +1545,10 @@
   // Deprecated 04/2025.
   registry->RegisterBooleanPref(
       kObsoleteUserAcknowledgedLocalPasswordsMigrationWarning, false);
+
+  // Deprecated 04/2025.
+  registry->RegisterIntegerPref(
+      kObsoleteLocalPasswordMigrationWarningPrefsVersion, 0);
 #endif
 }
 
@@ -2831,6 +2839,9 @@
   // Added 04/2025
   profile_prefs->ClearPref(
       kObsoleteUserAcknowledgedLocalPasswordsMigrationWarning);
+
+  // Added 04/2025.
+  profile_prefs->ClearPref(kObsoleteLocalPasswordMigrationWarningPrefsVersion);
 #endif
 
   // Please don't delete the following line. It is used by PRESUBMIT.py.