Removes filters as a way to provide gradients for IE<=9

- Lets us use rounded corners in IE9
 - Makes hover / active states work correctly in IE<=9

Only change for non-IE is that moving from hover -> not hovering
no longer animates (was a subtle blend) due to mismatch between the
bottom of the gradient and the background color.

Refactor: moves the primary button background to match the others.

docs/less.html

@@ -468,11 +468,6 @@
         <td><code>@opacity: 100</code></td>
         <td>Set, in whole numbers, the opacity percentage (e.g., "50" or "75")</td>
       </tr>
-      <tr>
-        <td><code>.reset-filter()</code></td>
-        <td><em class="muted">none</em></td>
-        <td>Resets any IE filter</td>
-      </tr>
     </tbody>
   </table>
   <h3>Forms</h3>

docs/templates/pages/less.mustache

@@ -403,11 +403,6 @@
         <td><code>@opacity: 100</code></td>
         <td>{{_i}}Set, in whole numbers, the opacity percentage (e.g., "50" or "75"){{/i}}</td>
       </tr>
-      <tr>
-        <td><code>.reset-filter()</code></td>
-        <td><em class="muted">none</em></td>
-        <td>{{_i}}Resets any IE filter{{/i}}</td>
-      </tr>
     </tbody>
   </table>
   <h3>Forms</h3>