Bugfix for "Open modal is shifting body content to the left" #9855

This solution do cover up the scrollbar of `body` with the scrollbar of `.modal`. So we have no shifting body content.

This solution removes the one property of class `.modal_open` and so this class seams obsolete.
May be the setter for this class should be removed in modal.js too.

A demo of this solution is here: http://jsbin.com/oHiPIJi/33