Fix display regression in the TODO Search Field

Currently, your GitHub Page for Flat UI shows this regression, apparently only manifesting itself on Retina displays:

![regression](https://dl.dropbox.com/s/kj928tyaeaxuznv/Screen%20Shot%202013-05-28%20at%207.29.17%20PM.png)

The reason for this seems to be that you changed that element to display an icon out of Flat-UI-Icons instead of using an image file:

```css
.todo-search:before {
  // ...
  font-family: 'Flat-UI-Icons';
  content: "\e01c";
  // ...
}
```

The Retina Display directive, however, has remained and the display is odd. Removing this line fixes the regression.

Signed-off-by: David Celis <me@davidcel.is>