gtkcssprovider: remove gtk_css_provider_get_default

This function is unused since cdc6e82720 commit and does not
contain fallback style.

https://gitlab.gnome.org/GNOME/gtk/issues/1226

docs/reference/gtk/gtk4-sections.txt

@@ -4935,7 +4935,6 @@ gtk_border_get_type
 <FILE>gtkcssprovider</FILE>
 <TITLE>GtkCssProvider</TITLE>
 GtkCssProvider
-gtk_css_provider_get_default
 gtk_css_provider_get_named
 gtk_css_provider_load_from_data
 gtk_css_provider_load_from_file

gtk/gtkcssprovider.c

@@ -1567,28 +1567,6 @@ gtk_css_provider_load_from_resource (GtkCssProvider *css_provider,
   g_object_unref (file);
 }
 
-/**
- * gtk_css_provider_get_default:
- *
- * Returns the provider containing the style settings used as a
- * fallback for all widgets.
- *
- * Returns: (transfer none): The provider used for fallback styling.
- *          This memory is owned by GTK+, and you must not free it.
- **/
-GtkCssProvider *
-gtk_css_provider_get_default (void)
-{
-  static GtkCssProvider *provider;
-
-  if (G_UNLIKELY (!provider))
-    {
-      provider = gtk_css_provider_new ();
-    }
-
-  return provider;
-}
-
 gchar *
 _gtk_get_theme_dir (void)
 {

gtk/gtkcssprovider.h

@@ -108,9 +108,6 @@ GDK_AVAILABLE_IN_ALL
 void             gtk_css_provider_load_from_resource (GtkCssProvider *css_provider,
                                                       const gchar    *resource_path);
 
-GDK_AVAILABLE_IN_ALL
-GtkCssProvider * gtk_css_provider_get_default (void);
-
 GDK_AVAILABLE_IN_ALL
 GtkCssProvider * gtk_css_provider_get_named (const gchar *name,
                                              const gchar *variant);