GEVER: Disabled icon for reference-prefix manage form.

The new action for the reference-prefix manager form should come without a icon.
![bildschirmfoto 2013-10-17 um 11 58 01](https://f.cloud.github.com/assets/485755/1350782/a0f6bb06-3712-11e3-824e-32ee8becfe6d.png)

@ninfaj or @shylux could you take a look?