Simplify and make more robust wxWebViewEdge initialization

Get rid of ms_isInitialized as it must be kept synchronized with
ms_loaderDll.IsLoaded() anyhow, and it's simpler to not have it at all
rather than ensuring this.

Also ensure that calling Initialize() again, after doing it first
unsuccessfully, doesn't assert because ms_loaderDll is already loaded,
by only leaving it with a valid handle if the initialization succeeded.

Closes #19041.

include/wx/msw/private/webview_edge.h

@@ -65,7 +65,6 @@ public:
 
     ICoreWebView2Settings* GetSettings();
 
-    static bool ms_isInitialized;
     static wxDynamicLibrary ms_loaderDll;
 
     static bool Initialize();