Revert "Fix showing a dialog multiple times (#12625)"

This reverts commit 2e78ebf.

src/cascadia/TerminalApp/AppLogic.cpp

@@ -348,11 +348,7 @@ namespace winrt::TerminalApp::implementation
         }
 
         _dialog = dialog;
-        // GH#12622: After the dialog is displayed, always clear it out. If we
-        // don't, we won't be able to display another!
-        const auto cleanup = wil::scope_exit([this]() {
-            _dialog = nullptr;
-        });
+
         // IMPORTANT: This is necessary as documented in the ContentDialog MSDN docs.
         // Since we're hosting the dialog in a Xaml island, we need to connect it to the
         // xaml tree somehow.