Remove check for is_markable_object in gc_mark_stack_values

gc_mark_and_pin already checks for is_markable_object.

gc.c

@@ -6217,9 +6217,7 @@ gc_mark_stack_values(rb_objspace_t *objspace, long n, const VALUE *values)
     long i;
 
     for (i=0; i<n; i++) {
-        if (is_markable_object(values[i])) {
-            gc_mark_and_pin(objspace, values[i]);
-        }
+        gc_mark_and_pin(objspace, values[i]);
     }
 }