Remove invalid `value` attribute from the share label

This fixes an HTML validation error introduced with the new share options.

```
From Jeff O'Brien:

Just received three of my sub-site checker results:

Husker Horn Studio | University of Nebraska–Lincoln
Music Education | University of Nebraska–Lincoln
Organ Studies | University of Nebraska–Lincoln

It looks like the share button that was added to the header late last week in the 4.0 template has a value tag that is being flagged by the checker. Since all of my pages are in 4.0, all of them have this error.
```

![screen shot 2014-02-17 at 8 05 35 am](https://f.cloud.github.com/assets/498678/2186140/335be780-97e1-11e3-87fa-573dbf6a2320.png)