WICKET-5616 CLONE - ModalWindow is not visible in Safari when opened from a link at the bottom of a large page

Revert https://github.com/apache/wicket/commit/61ebe2dbd55ff2ff5ac6f73909708021bb1acf4d as suggested by Luke Niesing and Jered Myers

(cherry picked from commit c70a5e00917503a98e27535c5057d1c8630d5060)