We were adding a .first CSS class to the portlet from JavaScript
instead of just using a smarter selector, which caused the incorrectly
styled portlet to visibly flash sometimes.
This was only useful for IE 6 (even IE 7 supports the sibling selector
'+', which we can use here), and we don't serve JavaScript to it
anymore. It would be neater to use ':first-of-type', but that's not as
widely supported.
Bug: T89542
Change-Id: I1904b3899c43bca52a6c996b8ce08c8bdf764aa0