libgimpwidgets: Fix formatting from 8adcc0cd

Some minor code style issues remained 
that I missed in 8adcc0cd.
After further reflection, I also converted
the code to be a private/internal function.
gimp_prop_check_button_new () should
cover the majority of GtkCheckButtons,
and the function is currently only used
to fix the size of those widgets. We could
revisit it as a public function in the
future if more use cases are found.

libgimpwidgets/gimpwidgetsutils.c

@@ -1279,23 +1279,3 @@ gimp_widget_set_handle_on_mapped (GtkWidget    *widget,
 
   return FALSE;
 }
-
-/* get the width of the label when it is bold */
-gint
-gimp_get_bold_label_width (const gchar *text)
-  {
-    GtkWidget      *temp_label = gtk_label_new (NULL);
-    GtkRequisition  natural_size;
-
-    gtk_label_set_text (GTK_LABEL (temp_label), text);
-    gtk_widget_set_visible (temp_label, TRUE);
-
-    gimp_label_set_attributes (GTK_LABEL (temp_label),
-                               PANGO_ATTR_WEIGHT, PANGO_WEIGHT_BOLD,
-                               -1);
-
-    gtk_widget_get_preferred_size (temp_label, NULL, &natural_size);
-    gtk_widget_destroy (temp_label);
-
-    return natural_size.width;
-}