the code `delete window.Modernizr` will throw an error in IE8

[anjianshi]

it throws object doesn't support this action see http://stackoverflow.com/questions/1073414/deleting-a-window-property-in-ie

[anjianshi]

may be our site or app should not support IE8, but we need to ensure modernizr's code won't break in old browsers, so that we can show a 'browser not support' message to user when tests not pass.