Core: Drop strundefined variable

Dropped `strundefined`, which simplifies the code, and saves a few gz bytes

```sh
Running "compare_size:files" (compare_size) task
   raw     gz Sizes
 57810  17534 dist/sizzle.js
 18375   7017 dist/sizzle.min.js

   raw     gz Compared to last run
   -39    -18 dist/sizzle.js
   +28     -8 dist/sizzle.min.js
```

Related: https://github.com/jquery/jquery/pull/1595