csslookup: Remove tautological if expression

A value is always either set or missing.
This was changed in a1f7c459b7, which
removed the ability for partial style computation.

gtk/gtkcsslookup.c

@@ -111,14 +111,11 @@ _gtk_css_lookup_resolve (GtkCssLookup      *lookup,
 
   for (i = 0; i < GTK_CSS_PROPERTY_N_PROPERTIES; i++)
     {
-      if (lookup->values[i].value ||
-          _gtk_bitmask_get (lookup->missing, i))
-        gtk_css_static_style_compute_value (style,
-                                            provider,
-                                            parent_style,
-                                            i,
-                                            lookup->values[i].value,
-                                            lookup->values[i].section);
-      /* else not a relevant property */
+      gtk_css_static_style_compute_value (style,
+                                          provider,
+                                          parent_style,
+                                          i,
+                                          lookup->values[i].value,
+                                          lookup->values[i].section);
     }
 }