Remove snippet expiration date from UI

Since expiration date of snippets was removed this also should be removed from UI. Currently it always says: `Expires: Never`

### Before
![screenshot 2014-01-25 08 59 59](https://f.cloud.github.com/assets/278301/2001229/6ee38e80-8598-11e3-8236-1456dc1ab617.png)

### After
![screenshot 2014-01-25 09 00 25](https://f.cloud.github.com/assets/278301/2001231/75b983d6-8598-11e3-96e6-0a11c0a722ca.png)