Remove cracy IE6 CSS notation in order to be compatible with LESS

If you try to include the current reset.css into LESS (in order to combine a bunch of CSS files), you get the error:

```
ParseError: Syntax Error on line 113 in /home/simon/HTML5-Reset/_/css/reset.css:113:11

112 /* prevent BG image flicker upon hover */
113 .ie6 html {filter: expression(document.execCommand("BackgroundImageCache", false, true));}
114
```

This is because LESS does not understand the cracy IE6 CSS notation.

Can we drop this filter in favor of LESS and probably other CSS pre-compilers?