remove unused credit card fields

Removes verification_value, issue_number, and start date from the payment admin, as they aren't used anywhere within spree and were always blank. Adds card_type, as that may be helpful.

Before:
![](http://i.hawth.ca/s/maJU9I70.png)

After:
![](http://i.hawth.ca/s/6yfh4tXt.png)