Fix scroll in modal

In administrator/index.php?option=com_menus&view=menus. Try clicking on a module in the dropdown and show the module settings in the modal. There is a horizontal scroll, since the position dropdown is too long:

![skrmbillede 2014-07-27 kl 13 23 38](https://cloud.githubusercontent.com/assets/1738811/3713726/8fa044f6-1580-11e4-9c2b-f94c79766284.png)
 
This PR removes input-xlarge from edit_positions.php. Which also has the side effect that the dropdown aligns nicer in the actual module view:

![skrmbillede 2014-07-27 kl 13 25 29](https://cloud.githubusercontent.com/assets/1738811/3713731/c3d35a6a-1580-11e4-9abf-717d6a010391.png)

Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33999