Revert "Fix build with newer Vala (#637)" (#659)

This reverts commit 9065ac6.

Vala has reverted the API break that necessitated this change, and now
we cannot build with newer Vala again.

Fixes #658

demo/Views/UtilsView.vala

@@ -73,11 +73,7 @@ public class UtilsView : Gtk.Grid {
             Granite.contrasting_foreground_color (bg_color).to_string ()
         );
 
-#if VALA_0_58
-        provider.load_from_data (css);
-#else
         provider.load_from_data ((uint8[])css);
-#endif
         demo_label_style_context.add_provider (
             provider,
             Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION

lib/Widgets/Utils.vala

@@ -263,11 +263,7 @@ namespace Granite.Widgets.Utils {
         var css = "@define-color color_primary %s;".printf (color.to_string ());
 
         var css_provider = new Gtk.CssProvider ();
-#if VALA_0_58
-        css_provider.load_from_data (css);
-#else
         css_provider.load_from_data (css.data);
-#endif
 
         Gtk.StyleContext.add_provider_for_display (Gdk.Display.get_default (), css_provider, priority);