Minor fixes to wxGTK3 with Broadway backend build.

Reuse some functions for both Broadway and X11 backends. Fix invalid casts
from Broadway screen/display types to X11 ones.

Closes #14469.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72079 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/utilsgtk.cpp

@@ -504,17 +504,3 @@ wxGUIAppTraits::GetStandardCmdLineOptions(wxArrayString& names,
 }
 
 #endif // __UNIX__
-
-#ifdef GDK_WINDOWING_BROADWAY
-
-bool wxLaunchDefaultApplication(const wxString& document, int flags)
-{
-    return false;
-}
-
-bool wxDoLaunchDefaultBrowser(const wxString& url, int flags)
-{
-    return false;
-}
-
-#endif