Clean up persisted windows after the feature is disabled (#18910)

I finally realized the missing piece.
We need to clear the state array unconditionally,
as otherwise it won't get cleared. Duh.

Closes #18584

## Validation Steps Performed
* Craft a state.json with a layout
* Launch Terminal while the feature is disabled
* state.json is cleaned up on exit ✅

Author: lhecker

src/cascadia/WindowsTerminal/WindowEmperor.cpp

@@ -925,9 +925,15 @@ void WindowEmperor::_finalizeSessionPersistence() const
 {
     const auto state = ApplicationState::SharedInstance();
 
-    if (_forcePersistence || _app.Logic().Settings().GlobalSettings().ShouldUsePersistedLayout())
+    // Calling an `ApplicationState` setter triggers a write to state.json.
+    // With this if condition we avoid an unnecessary write when persistence is disabled.
+    if (state.PersistedWindowLayouts())
     {
         state.PersistedWindowLayouts(nullptr);
+    }
+
+    if (_forcePersistence || _app.Logic().Settings().GlobalSettings().ShouldUsePersistedLayout())
+    {
         for (const auto& w : _windows)
         {
             w->Logic().PersistState();