[shortstand] Remove the flag kStandaloneWindowMigrationUx

The flag kStandaloneWindowMigrationUx is dependent on kCrosShortstand
(see IsStandaloneWindowMigrationUxEnabled() definition),
and we are in the process of deleting kCrosShortstand.
This CL rolls back http://crrev.com/c/5039996 and http://crrev.com/c/5087049
Also deleted the pref kStandaloneWindowMigrationNudgeShown.

BUG=b:338393239

Change-Id: Ifdb42493cbe4373bdf7daa33da124d2a3a5f99f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5752525
Reviewed-by: Scott Violet <[email protected]>
Auto-Submit: Oleg Nitz <[email protected]>
Reviewed-by: Jiaming Cheng <[email protected]>
Reviewed-by: Rushan Suleymanov <[email protected]>
Commit-Queue: Oleg Nitz <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1336549}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index 4da3cf2..6636e50 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -1031,6 +1031,12 @@
 constexpr char kShowTunaScreenEnabled[] = "ash.tuna_screen_oobe_enabled";
 #endif
 
+// Deprecated 08/2024
+#if BUILDFLAG(IS_CHROMEOS)
+inline constexpr char kStandaloneWindowMigrationNudgeShown[] =
+    "standalone_window_migration_nudge_shown";
+#endif
+
 // Register local state used only for migration (clearing or moving to a new
 // key).
 void RegisterLocalStatePrefsForMigration(PrefRegistrySimple* registry) {
@@ -1415,6 +1421,11 @@
   // Deprecated 07/2024.
   registry->RegisterBooleanPref(kShowTunaScreenEnabled, true);
 #endif  // BUILDFLAG(IS_CHROMEOS_ASH)
+
+// Deprecated 08/2024
+#if BUILDFLAG(IS_CHROMEOS)
+  registry->RegisterBooleanPref(kStandaloneWindowMigrationNudgeShown, false);
+#endif
 }
 
 void ClearSyncRequestedPrefAndMaybeMigrate(PrefService* profile_prefs) {
@@ -2708,6 +2719,11 @@
   profile_prefs->ClearPref(kShowTunaScreenEnabled);
 #endif
 
+// Added 08/2024
+#if BUILDFLAG(IS_CHROMEOS)
+  profile_prefs->ClearPref(kStandaloneWindowMigrationNudgeShown);
+#endif
+
   // Please don't delete the following line. It is used by PRESUBMIT.py.
   // END_MIGRATE_OBSOLETE_PROFILE_PREFS