Fix clear button in discover view

In the latest staging version it's not possible to clear the search input field in com_installer&view=discover, since it relies on Mootools, which isn't loaded in that view. This PR fixes it.

![skaermbillede 2014-12-19 kl 19 46 43](https://cloud.githubusercontent.com/assets/1738811/5509408/c9eb10c4-87b7-11e4-98f3-81bb08b5c473.png)