[ticket/12437] Clean up redundant "clear" & "corners" elements 

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.

PHPBB3-12437