Clean up redundant "clear" elements & "corners"

Since we are dropping IE6 & IE7 support, it would be nice to get rid of some redundant style elements like <span class="clear"></span>.

They don't add anything if they are inside container with a clearing rule like:

.inner:after {

    clear: both;

    content: "";

    display: block;

}

Since IE8 supports this method, we could simply remove these redundant elements.

Additionally, there are still remnants of old "corners-top" and "corners-bottom" code, which can easily be removed.