Tweaked Line Height of comment delete buttons to center the text better

I almost left this for someone else, but it was driving me nuts...
This makes the `x` in the delete button more centered vertically.

Before
![delete_button_before](https://f.cloud.github.com/assets/315948/2162006/0ffb2984-94cd-11e3-9d22-11cf9e4edc81.png)
After
![delete_button_after](https://f.cloud.github.com/assets/315948/2162008/11e78102-94cd-11e3-93fc-6887fe932478.png)