Drop XamlApplication::EnsureCreated (#15509)

This function is an implementation detail and shouldn't be exposed in the
public interface.  This was missed out by PR #15407.

A module requiring Xaml support can specify it using the bool property
`IReactViewComponentBuilder.XamlSupport`.

Author: sundaramramaswamy

change/react-native-windows-f9a025e2-d610-41fb-9e23-591080c8e249.json

@@ -0,0 +1,7 @@
+{
+  "type": "none",
+  "comment": "Drop XamlApplication::EnsureCreated",
+  "packageName": "react-native-windows",
+  "email": "email not defined",
+  "dependentChangeType": "none"
+}

vnext/Microsoft.ReactNative/XamlApplication.idl

@@ -8,9 +8,6 @@ namespace Microsoft.ReactNative.Xaml {
   DOC_STRING("Initializes a new XamlApplication instance for RNW-hosted WinUI/XAML.")
   XamlApplication();
 
-  DOC_STRING("Ensures the XamlApplication singleton exists.")
-  static void EnsureCreated();
-
   DOC_STRING("Gets the process-wide XamlApplication singleton.")
   static XamlApplication Current {
     get;