While you can't comment it out, you can define the text as a zero length string by removing the definitions content.

To do this, change...

PHP Code:
define('TEXT_SELECTED_BILLING_DESTINATION', 'Your billing address is shown to the left. The billing address should match the address on your credit card statement. You can change the billing address by clicking the <em>Change Address</em> button.'); 
...to...

PHP Code:
define('TEXT_SELECTED_BILLING_DESTINATION', ''); 
you will still have the whitespace left behind, but it might suit your purpose if you have trouble with the other option?

cheers
andrejs