Colorbox CSS - Remove IE6 hack.

Remove support for IE6 transparency hack. We don't include the required images anyway (so wasn't working). Plus: http://www.ie6countdown.com/

Signed-off-by: Andrew <dent.ace@gmail.com>
Merge pull request #1013 from ace-dent/feature-css_tidy2

Colorbox CSS - Remove IE6 hack.